diff --git a/contrib/hstore_plpython/expected/hstore_plpython.out b/contrib/hstore_plpython/expected/hstore_plpython.out
index 57d83fa2db5..b1837715ce5 100644
--- a/contrib/hstore_plpython/expected/hstore_plpython.out
+++ b/contrib/hstore_plpython/expected/hstore_plpython.out
@@ -43,6 +43,71 @@ SELECT test1bad();
 ERROR:  not a Python mapping
 CONTEXT:  while creating return value
 PL/Python function "test1bad"
+-- A mapping whose items() raises should be reported as an error, not crash
+-- the backend
+CREATE FUNCTION test1broken() RETURNS hstore
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE hstore
+AS $$
+class C(dict):
+    def items(self):
+        raise ValueError('items failed')
+d = C()
+d['x'] = 1
+return d
+$$;
+SELECT test1broken();
+ERROR:  could not get items from Python mapping
+CONTEXT:  while creating return value
+PL/Python function "test1broken"
+-- Likewise for a mapping whose items() does not return key/value pairs
+CREATE FUNCTION test1malformed() RETURNS hstore
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE hstore
+AS $$
+class C(dict):
+    def items(self):
+        return [42]
+d = C()
+d['x'] = 1
+return d
+$$;
+SELECT test1malformed();
+ERROR:  items() of a Python mapping must return key/value pairs
+CONTEXT:  while creating return value
+PL/Python function "test1malformed"
+-- Likewise for a mapping whose items() yields fewer pairs than its length
+CREATE FUNCTION test1short() RETURNS hstore
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE hstore
+AS $$
+class C(dict):
+    def items(self):
+        return []
+d = C()
+d['x'] = 1
+return d
+$$;
+SELECT test1short();
+ERROR:  items() of a Python mapping must return key/value pairs
+CONTEXT:  while creating return value
+PL/Python function "test1short"
+-- Likewise for a mapping whose __len__() raises
+CREATE FUNCTION test1brokenlen() RETURNS hstore
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE hstore
+AS $$
+class C(dict):
+    def __len__(self):
+        raise ValueError('len failed')
+d = C()
+d['x'] = 1
+return d
+$$;
+SELECT test1brokenlen();
+ERROR:  could not get size of Python mapping
+CONTEXT:  while creating return value
+PL/Python function "test1brokenlen"
 -- test hstore[] -> python
 CREATE FUNCTION test1arr(val hstore[]) RETURNS int
 LANGUAGE plpythonu
diff --git a/contrib/hstore_plpython/hstore_plpython.c b/contrib/hstore_plpython/hstore_plpython.c
index 2d144043abd..db67bb3685a 100644
--- a/contrib/hstore_plpython/hstore_plpython.c
+++ b/contrib/hstore_plpython/hstore_plpython.c
@@ -145,7 +145,16 @@ plpython_to_hstore(PG_FUNCTION_ARGS)
 				 errmsg("not a Python mapping")));
 
 	pcount = PyMapping_Size(dict);
+	if (pcount < 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_DATATYPE_MISMATCH),
+				 errmsg("could not get size of Python mapping")));
+
 	items = PyMapping_Items(dict);
+	if (items == NULL)
+		ereport(ERROR,
+				(errcode(ERRCODE_DATATYPE_MISMATCH),
+				 errmsg("could not get items from Python mapping")));
 
 	PG_TRY();
 	{
@@ -162,6 +171,13 @@ plpython_to_hstore(PG_FUNCTION_ARGS)
 			PyObject   *value;
 
 			tuple = PyList_GetItem(items, i);
+
+			/* The mapping's items() must yield key/value pairs */
+			if (tuple == NULL || !PyTuple_Check(tuple) || PyTuple_Size(tuple) < 2)
+				ereport(ERROR,
+						(errcode(ERRCODE_DATATYPE_MISMATCH),
+						 errmsg("items() of a Python mapping must return key/value pairs")));
+
 			key = PyTuple_GetItem(tuple, 0);
 			value = PyTuple_GetItem(tuple, 1);
 
diff --git a/contrib/hstore_plpython/sql/hstore_plpython.sql b/contrib/hstore_plpython/sql/hstore_plpython.sql
index 1aa4416512a..8548b7dbe03 100644
--- a/contrib/hstore_plpython/sql/hstore_plpython.sql
+++ b/contrib/hstore_plpython/sql/hstore_plpython.sql
@@ -38,6 +38,71 @@ $$;
 SELECT test1bad();
 
 
+-- A mapping whose items() raises should be reported as an error, not crash
+-- the backend
+CREATE FUNCTION test1broken() RETURNS hstore
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE hstore
+AS $$
+class C(dict):
+    def items(self):
+        raise ValueError('items failed')
+d = C()
+d['x'] = 1
+return d
+$$;
+
+SELECT test1broken();
+
+
+-- Likewise for a mapping whose items() does not return key/value pairs
+CREATE FUNCTION test1malformed() RETURNS hstore
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE hstore
+AS $$
+class C(dict):
+    def items(self):
+        return [42]
+d = C()
+d['x'] = 1
+return d
+$$;
+
+SELECT test1malformed();
+
+
+-- Likewise for a mapping whose items() yields fewer pairs than its length
+CREATE FUNCTION test1short() RETURNS hstore
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE hstore
+AS $$
+class C(dict):
+    def items(self):
+        return []
+d = C()
+d['x'] = 1
+return d
+$$;
+
+SELECT test1short();
+
+
+-- Likewise for a mapping whose __len__() raises
+CREATE FUNCTION test1brokenlen() RETURNS hstore
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE hstore
+AS $$
+class C(dict):
+    def __len__(self):
+        raise ValueError('len failed')
+d = C()
+d['x'] = 1
+return d
+$$;
+
+SELECT test1brokenlen();
+
+
 -- test hstore[] -> python
 CREATE FUNCTION test1arr(val hstore[]) RETURNS int
 LANGUAGE plpythonu
diff --git a/contrib/jsonb_plpython/expected/jsonb_plpython.out b/contrib/jsonb_plpython/expected/jsonb_plpython.out
index b491fe9cc68..f9bde492095 100644
--- a/contrib/jsonb_plpython/expected/jsonb_plpython.out
+++ b/contrib/jsonb_plpython/expected/jsonb_plpython.out
@@ -304,3 +304,92 @@ SELECT test_dict1();
  {"": 2, "a": 1, "33": 3}
 (1 row)
 
+-- A custom sequence whose __getitem__ raises should be reported as an error,
+-- not crash the backend
+CREATE FUNCTION test_broken_sequence() RETURNS jsonb
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE jsonb
+AS $$
+class C:
+    def __len__(self):
+        return 2
+    def __getitem__(self, i):
+        raise ValueError('getitem failed')
+return C()
+$$;
+SELECT test_broken_sequence();
+ERROR:  could not get element 0 from sequence
+DETAIL:  ValueError: getitem failed
+CONTEXT:  Traceback (most recent call last):
+while creating return value
+PL/Python function "test_broken_sequence"
+-- A mapping whose items() raises should be reported as an error, not crash
+-- the backend
+CREATE FUNCTION test_broken_mapping() RETURNS jsonb
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE jsonb
+AS $$
+class C(dict):
+    def items(self):
+        raise ValueError('items failed')
+d = C()
+d['x'] = 1
+return d
+$$;
+SELECT test_broken_mapping();
+ERROR:  could not get items from Python mapping
+DETAIL:  ValueError: items failed
+CONTEXT:  Traceback (most recent call last):
+while creating return value
+PL/Python function "test_broken_mapping"
+-- Likewise for a mapping whose items() does not return key/value pairs
+CREATE FUNCTION test_malformed_mapping() RETURNS jsonb
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE jsonb
+AS $$
+class C(dict):
+    def items(self):
+        return [42]
+d = C()
+d['x'] = 1
+return d
+$$;
+SELECT test_malformed_mapping();
+ERROR:  items() of a Python mapping must return key/value pairs
+CONTEXT:  while creating return value
+PL/Python function "test_malformed_mapping"
+-- Likewise for a mapping whose items() yields fewer pairs than its length
+CREATE FUNCTION test_short_mapping() RETURNS jsonb
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE jsonb
+AS $$
+class C(dict):
+    def items(self):
+        return []
+d = C()
+d['x'] = 1
+return d
+$$;
+SELECT test_short_mapping();
+ERROR:  items() of a Python mapping must return key/value pairs
+DETAIL:  IndexError: list index out of range
+CONTEXT:  while creating return value
+PL/Python function "test_short_mapping"
+-- Likewise for a mapping whose __len__() raises
+CREATE FUNCTION test_broken_len_mapping() RETURNS jsonb
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE jsonb
+AS $$
+class C(dict):
+    def __len__(self):
+        raise ValueError('len failed')
+d = C()
+d['x'] = 1
+return d
+$$;
+SELECT test_broken_len_mapping();
+ERROR:  could not get size of Python mapping
+DETAIL:  ValueError: len failed
+CONTEXT:  Traceback (most recent call last):
+while creating return value
+PL/Python function "test_broken_len_mapping"
diff --git a/contrib/jsonb_plpython/jsonb_plpython.c b/contrib/jsonb_plpython/jsonb_plpython.c
index 8d95f09f23d..1c7097a4075 100644
--- a/contrib/jsonb_plpython/jsonb_plpython.c
+++ b/contrib/jsonb_plpython/jsonb_plpython.c
@@ -277,7 +277,12 @@ PLyMapping_ToJsonbValue(PyObject *obj, JsonbParseState **jsonb_state)
 	JsonbValue *volatile out;
 
 	pcount = PyMapping_Size(obj);
+	if (pcount < 0)
+		PLy_elog(ERROR, "could not get size of Python mapping");
+
 	items = PyMapping_Items(obj);
+	if (items == NULL)
+		PLy_elog(ERROR, "could not get items from Python mapping");
 
 	PG_TRY();
 	{
@@ -289,8 +294,15 @@ PLyMapping_ToJsonbValue(PyObject *obj, JsonbParseState **jsonb_state)
 		{
 			JsonbValue	jbvKey;
 			PyObject   *item = PyList_GetItem(items, i);
-			PyObject   *key = PyTuple_GetItem(item, 0);
-			PyObject   *value = PyTuple_GetItem(item, 1);
+			PyObject   *key;
+			PyObject   *value;
+
+			/* The mapping's items() must yield key/value pairs */
+			if (item == NULL || !PyTuple_Check(item) || PyTuple_Size(item) < 2)
+				PLy_elog(ERROR, "items() of a Python mapping must return key/value pairs");
+
+			key = PyTuple_GetItem(item, 0);
+			value = PyTuple_GetItem(item, 1);
 
 			/* Python dictionary can have None as key */
 			if (key == Py_None)
@@ -342,7 +354,10 @@ PLySequence_ToJsonbValue(PyObject *obj, JsonbParseState **jsonb_state)
 		for (i = 0; i < pcount; i++)
 		{
 			value = PySequence_GetItem(obj, i);
-			Assert(value);
+
+			/* PySequence_GetItem() can return NULL, with an exception set */
+			if (value == NULL)
+				PLy_elog(ERROR, "could not get element %d from sequence", (int) i);
 
 			(void) PLyObject_ToJsonbValue(value, jsonb_state, true);
 			Py_XDECREF(value);
diff --git a/contrib/jsonb_plpython/sql/jsonb_plpython.sql b/contrib/jsonb_plpython/sql/jsonb_plpython.sql
index 2ee1bca0a98..78578fa3d8a 100644
--- a/contrib/jsonb_plpython/sql/jsonb_plpython.sql
+++ b/contrib/jsonb_plpython/sql/jsonb_plpython.sql
@@ -181,3 +181,80 @@ return x
 $$;
 
 SELECT test_dict1();
+
+-- A custom sequence whose __getitem__ raises should be reported as an error,
+-- not crash the backend
+CREATE FUNCTION test_broken_sequence() RETURNS jsonb
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE jsonb
+AS $$
+class C:
+    def __len__(self):
+        return 2
+    def __getitem__(self, i):
+        raise ValueError('getitem failed')
+return C()
+$$;
+
+SELECT test_broken_sequence();
+
+-- A mapping whose items() raises should be reported as an error, not crash
+-- the backend
+CREATE FUNCTION test_broken_mapping() RETURNS jsonb
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE jsonb
+AS $$
+class C(dict):
+    def items(self):
+        raise ValueError('items failed')
+d = C()
+d['x'] = 1
+return d
+$$;
+
+SELECT test_broken_mapping();
+
+-- Likewise for a mapping whose items() does not return key/value pairs
+CREATE FUNCTION test_malformed_mapping() RETURNS jsonb
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE jsonb
+AS $$
+class C(dict):
+    def items(self):
+        return [42]
+d = C()
+d['x'] = 1
+return d
+$$;
+
+SELECT test_malformed_mapping();
+
+-- Likewise for a mapping whose items() yields fewer pairs than its length
+CREATE FUNCTION test_short_mapping() RETURNS jsonb
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE jsonb
+AS $$
+class C(dict):
+    def items(self):
+        return []
+d = C()
+d['x'] = 1
+return d
+$$;
+
+SELECT test_short_mapping();
+
+-- Likewise for a mapping whose __len__() raises
+CREATE FUNCTION test_broken_len_mapping() RETURNS jsonb
+LANGUAGE plpython3u
+TRANSFORM FOR TYPE jsonb
+AS $$
+class C(dict):
+    def __len__(self):
+        raise ValueError('len failed')
+d = C()
+d['x'] = 1
+return d
+$$;
+
+SELECT test_broken_len_mapping();
diff --git a/src/pl/plpython/expected/plpython_composite.out b/src/pl/plpython/expected/plpython_composite.out
index b9111210d54..a860dd24a57 100644
--- a/src/pl/plpython/expected/plpython_composite.out
+++ b/src/pl/plpython/expected/plpython_composite.out
@@ -606,3 +606,19 @@ DETAIL:  Missing left parenthesis.
 HINT:  To return a composite type in an array, return the composite type as a Python tuple, e.g., "[('foo',)]".
 CONTEXT:  while creating return value
 PL/Python function "composite_type_as_list_broken"
+-- A custom sequence whose length matches the tuple but whose __getitem__
+-- raises should be reported as an error, not crash the backend.
+CREATE FUNCTION composite_type_as_broken_sequence() RETURNS type_record AS $$
+class C:
+    def __len__(self):
+        return 2
+    def __getitem__(self, i):
+        raise ValueError('getitem failed')
+return C()
+$$ LANGUAGE plpython3u;
+SELECT * FROM composite_type_as_broken_sequence();
+ERROR:  could not get element 0 from sequence
+DETAIL:  ValueError: getitem failed
+CONTEXT:  Traceback (most recent call last):
+while creating return value
+PL/Python function "composite_type_as_broken_sequence"
diff --git a/src/pl/plpython/expected/plpython_spi.out b/src/pl/plpython/expected/plpython_spi.out
index a09df68c7d1..eeed3d25a7b 100644
--- a/src/pl/plpython/expected/plpython_spi.out
+++ b/src/pl/plpython/expected/plpython_spi.out
@@ -464,3 +464,54 @@ SELECT plan_composite_args();
  (3,label)
 (1 row)
 
+-- A custom argument sequence whose length matches the plan but whose
+-- __getitem__ raises should be reported as an error, not crash the backend.
+CREATE FUNCTION plan_broken_arg_sequence() RETURNS void AS $$
+plan = plpy.prepare("select $1", ["int4"])
+class C:
+    def __len__(self):
+        return 1
+    def __getitem__(self, i):
+        raise ValueError('getitem failed')
+plpy.execute(plan, C())
+$$ LANGUAGE plpython3u;
+SELECT plan_broken_arg_sequence();
+ERROR:  spiexceptions.ExternalRoutineException: could not get element 0 from sequence
+DETAIL:  ValueError: getitem failed
+CONTEXT:  Traceback (most recent call last):
+  PL/Python function "plan_broken_arg_sequence", line 8, in <module>
+    plpy.execute(plan, C())
+PL/Python function "plan_broken_arg_sequence"
+-- Likewise for the type-name list passed to plpy.prepare().
+CREATE FUNCTION prepare_broken_type_sequence() RETURNS void AS $$
+class C:
+    def __len__(self):
+        return 1
+    def __getitem__(self, i):
+        raise ValueError('getitem failed')
+plpy.prepare("select $1", C())
+$$ LANGUAGE plpython3u;
+SELECT prepare_broken_type_sequence();
+ERROR:  spiexceptions.ExternalRoutineException: could not get element 0 from sequence
+DETAIL:  ValueError: getitem failed
+CONTEXT:  Traceback (most recent call last):
+  PL/Python function "prepare_broken_type_sequence", line 7, in <module>
+    plpy.prepare("select $1", C())
+PL/Python function "prepare_broken_type_sequence"
+-- Likewise for the argument sequence passed to plpy.cursor().
+CREATE FUNCTION cursor_broken_arg_sequence() RETURNS void AS $$
+plan = plpy.prepare("select $1", ["int4"])
+class C:
+    def __len__(self):
+        return 1
+    def __getitem__(self, i):
+        raise ValueError('getitem failed')
+plpy.cursor(plan, C())
+$$ LANGUAGE plpython3u;
+SELECT cursor_broken_arg_sequence();
+ERROR:  spiexceptions.ExternalRoutineException: could not get element 0 from sequence
+DETAIL:  ValueError: getitem failed
+CONTEXT:  Traceback (most recent call last):
+  PL/Python function "cursor_broken_arg_sequence", line 8, in <module>
+    plpy.cursor(plan, C())
+PL/Python function "cursor_broken_arg_sequence"
diff --git a/src/pl/plpython/expected/plpython_types.out b/src/pl/plpython/expected/plpython_types.out
index d1776365c35..6ef55a4efcc 100644
--- a/src/pl/plpython/expected/plpython_types.out
+++ b/src/pl/plpython/expected/plpython_types.out
@@ -796,6 +796,22 @@ SELECT * FROM test_type_conversion_array_error();
 ERROR:  return value of function with array return type is not a Python sequence
 CONTEXT:  while creating return value
 PL/Python function "test_type_conversion_array_error"
+-- A custom sequence whose __getitem__ raises should be reported as an error,
+-- not crash the backend.
+CREATE FUNCTION test_type_conversion_array_getitem_fail() RETURNS int[] AS $$
+class C:
+    def __len__(self):
+        return 2
+    def __getitem__(self, i):
+        raise ValueError('getitem failed')
+return C()
+$$ LANGUAGE plpython3u;
+SELECT * FROM test_type_conversion_array_getitem_fail();
+ERROR:  could not get element 0 from sequence
+DETAIL:  ValueError: getitem failed
+CONTEXT:  Traceback (most recent call last):
+while creating return value
+PL/Python function "test_type_conversion_array_getitem_fail"
 --
 -- Domains over arrays
 --
diff --git a/src/pl/plpython/expected/plpython_types_3.out b/src/pl/plpython/expected/plpython_types_3.out
index 6fadd5462d1..ce362cbbc3c 100644
--- a/src/pl/plpython/expected/plpython_types_3.out
+++ b/src/pl/plpython/expected/plpython_types_3.out
@@ -796,6 +796,22 @@ SELECT * FROM test_type_conversion_array_error();
 ERROR:  return value of function with array return type is not a Python sequence
 CONTEXT:  while creating return value
 PL/Python function "test_type_conversion_array_error"
+-- A custom sequence whose __getitem__ raises should be reported as an error,
+-- not crash the backend.
+CREATE FUNCTION test_type_conversion_array_getitem_fail() RETURNS int[] AS $$
+class C:
+    def __len__(self):
+        return 2
+    def __getitem__(self, i):
+        raise ValueError('getitem failed')
+return C()
+$$ LANGUAGE plpython3u;
+SELECT * FROM test_type_conversion_array_getitem_fail();
+ERROR:  could not get element 0 from sequence
+DETAIL:  ValueError: getitem failed
+CONTEXT:  Traceback (most recent call last):
+while creating return value
+PL/Python function "test_type_conversion_array_getitem_fail"
 --
 -- Domains over arrays
 --
diff --git a/src/pl/plpython/plpy_cursorobject.c b/src/pl/plpython/plpy_cursorobject.c
index d783819e82a..06229fcd763 100644
--- a/src/pl/plpython/plpy_cursorobject.c
+++ b/src/pl/plpython/plpy_cursorobject.c
@@ -231,6 +231,11 @@ PLy_cursor_plan(PyObject *ob, PyObject *args)
 			PyObject   *elem;
 
 			elem = PySequence_GetItem(args, j);
+
+			/* PySequence_GetItem() can return NULL, with an exception set */
+			if (elem == NULL)
+				PLy_elog(ERROR, "could not get element %d from sequence", j);
+
 			PG_TRY();
 			{
 				bool		isnull;
diff --git a/src/pl/plpython/plpy_spi.c b/src/pl/plpython/plpy_spi.c
index 53af9c05efa..594f0a67ab7 100644
--- a/src/pl/plpython/plpy_spi.c
+++ b/src/pl/plpython/plpy_spi.c
@@ -90,6 +90,11 @@ PLy_spi_prepare(PyObject *self, PyObject *args)
 			int32		typmod;
 
 			optr = PySequence_GetItem(list, i);
+
+			/* PySequence_GetItem() can return NULL, with an exception set */
+			if (optr == NULL)
+				PLy_elog(ERROR, "could not get element %d from sequence", i);
+
 			if (PyString_Check(optr))
 				sptr = PyString_AsString(optr);
 			else if (PyUnicode_Check(optr))
@@ -254,6 +259,11 @@ PLy_spi_execute_plan(PyObject *ob, PyObject *list, long limit)
 			PyObject   *elem;
 
 			elem = PySequence_GetItem(list, j);
+
+			/* PySequence_GetItem() can return NULL, with an exception set */
+			if (elem == NULL)
+				PLy_elog(ERROR, "could not get element %d from sequence", j);
+
 			PG_TRY();
 			{
 				bool		isnull;
diff --git a/src/pl/plpython/plpy_typeio.c b/src/pl/plpython/plpy_typeio.c
index 707fe416b2f..35be8bfd8ab 100644
--- a/src/pl/plpython/plpy_typeio.c
+++ b/src/pl/plpython/plpy_typeio.c
@@ -1214,6 +1214,10 @@ PLySequence_ToArray_recurse(PyObject *obj, ArrayBuildState **astatep,
 		/* fetch the array element */
 		PyObject   *subobj = PySequence_GetItem(obj, i);
 
+		/* PySequence_GetItem() can return NULL, with an exception set */
+		if (subobj == NULL)
+			PLy_elog(ERROR, "could not get element %d from sequence", i);
+
 		/* need PG_TRY to ensure we release the subobj's refcount */
 		PG_TRY();
 		{
@@ -1460,7 +1464,10 @@ PLySequence_ToComposite(PLyObToDatum *arg, TupleDesc desc, PyObject *sequence)
 		PG_TRY();
 		{
 			value = PySequence_GetItem(sequence, idx);
-			Assert(value);
+
+			/* PySequence_GetItem() can return NULL, with an exception set */
+			if (value == NULL)
+				PLy_elog(ERROR, "could not get element %d from sequence", idx);
 
 			values[i] = att->func(att, value, &nulls[i], false);
 
diff --git a/src/pl/plpython/sql/plpython_composite.sql b/src/pl/plpython/sql/plpython_composite.sql
index 844b761bf11..d7ac6463c1c 100644
--- a/src/pl/plpython/sql/plpython_composite.sql
+++ b/src/pl/plpython/sql/plpython_composite.sql
@@ -233,3 +233,15 @@ CREATE FUNCTION composite_type_as_list_broken()  RETURNS type_record[] AS $$
   return [['first', 1]];
 $$ LANGUAGE plpythonu;
 SELECT * FROM composite_type_as_list_broken();
+
+-- A custom sequence whose length matches the tuple but whose __getitem__
+-- raises should be reported as an error, not crash the backend.
+CREATE FUNCTION composite_type_as_broken_sequence() RETURNS type_record AS $$
+class C:
+    def __len__(self):
+        return 2
+    def __getitem__(self, i):
+        raise ValueError('getitem failed')
+return C()
+$$ LANGUAGE plpython3u;
+SELECT * FROM composite_type_as_broken_sequence();
diff --git a/src/pl/plpython/sql/plpython_spi.sql b/src/pl/plpython/sql/plpython_spi.sql
index dd77833ed56..93d2b0d90cd 100644
--- a/src/pl/plpython/sql/plpython_spi.sql
+++ b/src/pl/plpython/sql/plpython_spi.sql
@@ -320,3 +320,42 @@ SELECT cursor_fetch_next_empty();
 SELECT cursor_plan();
 SELECT cursor_plan_wrong_args();
 SELECT plan_composite_args();
+
+-- A custom argument sequence whose length matches the plan but whose
+-- __getitem__ raises should be reported as an error, not crash the backend.
+CREATE FUNCTION plan_broken_arg_sequence() RETURNS void AS $$
+plan = plpy.prepare("select $1", ["int4"])
+class C:
+    def __len__(self):
+        return 1
+    def __getitem__(self, i):
+        raise ValueError('getitem failed')
+plpy.execute(plan, C())
+$$ LANGUAGE plpython3u;
+
+SELECT plan_broken_arg_sequence();
+
+-- Likewise for the type-name list passed to plpy.prepare().
+CREATE FUNCTION prepare_broken_type_sequence() RETURNS void AS $$
+class C:
+    def __len__(self):
+        return 1
+    def __getitem__(self, i):
+        raise ValueError('getitem failed')
+plpy.prepare("select $1", C())
+$$ LANGUAGE plpython3u;
+
+SELECT prepare_broken_type_sequence();
+
+-- Likewise for the argument sequence passed to plpy.cursor().
+CREATE FUNCTION cursor_broken_arg_sequence() RETURNS void AS $$
+plan = plpy.prepare("select $1", ["int4"])
+class C:
+    def __len__(self):
+        return 1
+    def __getitem__(self, i):
+        raise ValueError('getitem failed')
+plpy.cursor(plan, C())
+$$ LANGUAGE plpython3u;
+
+SELECT cursor_broken_arg_sequence();
diff --git a/src/pl/plpython/sql/plpython_types.sql b/src/pl/plpython/sql/plpython_types.sql
index a8ceb661113..b64fac4d17f 100644
--- a/src/pl/plpython/sql/plpython_types.sql
+++ b/src/pl/plpython/sql/plpython_types.sql
@@ -417,6 +417,19 @@ $$ LANGUAGE plpythonu;
 
 SELECT * FROM test_type_conversion_array_error();
 
+-- A custom sequence whose __getitem__ raises should be reported as an error,
+-- not crash the backend.
+CREATE FUNCTION test_type_conversion_array_getitem_fail() RETURNS int[] AS $$
+class C:
+    def __len__(self):
+        return 2
+    def __getitem__(self, i):
+        raise ValueError('getitem failed')
+return C()
+$$ LANGUAGE plpython3u;
+
+SELECT * FROM test_type_conversion_array_getitem_fail();
+
 
 --
 -- Domains over arrays