diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 27fd7e9720a..a5b4bc5b7d5 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -2586,9 +2586,23 @@ ExtendBufferedRelCommon(BufferManagerRelation bmr,
 										 extend_by);
 
 	if (bmr.relpersistence == RELPERSISTENCE_TEMP)
+	{
+		/*
+		 * Reject attempts to extend non-local temporary relations; we have no
+		 * ability to transfer about-to-be-created local buffers into the
+		 * owning session's local buffers.  This is the canonical place for
+		 * the check, covering any attempt to extend a non-local temporary
+		 * relation.
+		 */
+		if (bmr.rel && RELATION_IS_OTHER_TEMP(bmr.rel))
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("cannot access temporary tables of other sessions")));
+
 		first_block = ExtendBufferedRelLocal(bmr, fork, flags,
 											 extend_by, extend_upto,
 											 buffers, &extend_by);
+	}
 	else
 		first_block = ExtendBufferedRelShared(bmr, fork, strategy, flags,
 											  extend_by, extend_upto,
diff --git a/src/include/utils/rel.h b/src/include/utils/rel.h
index 2f86c79a907..09c794239ca 100644
--- a/src/include/utils/rel.h
+++ b/src/include/utils/rel.h
@@ -668,10 +668,10 @@ RelationCloseSmgr(Relation relation)
  * the owning session keeps the data in its private local buffer pool,
  * which we cannot access.  Existing buffer-manager entry points
  * (ReadBuffer_common(), StartReadBuffersImpl(), read_stream_begin_impl(),
- * and PrefetchBuffer()) already enforce this; any new buffer-access entry
- * points must do the same.  Command-level code (TRUNCATE, ALTER TABLE,
- * VACUUM, CLUSTER, REINDEX, ...) additionally uses this macro for
- * command-specific error messages.
+ * PrefetchBuffer() and ExtendBufferedRelCommon()) already enforce this; any
+ * new buffer-access entry points must do the same.  Command-level code
+ * (TRUNCATE, ALTER TABLE, VACUUM, CLUSTER, REINDEX, ...) additionally uses
+ * this macro for command-specific error messages.
  *
  * Beware of multiple eval of argument
  */
diff --git a/src/test/modules/test_misc/t/013_temp_obj_multisession.pl b/src/test/modules/test_misc/t/013_temp_obj_multisession.pl
index c56a032e57f..0e28ed59d05 100644
--- a/src/test/modules/test_misc/t/013_temp_obj_multisession.pl
+++ b/src/test/modules/test_misc/t/013_temp_obj_multisession.pl
@@ -37,6 +37,10 @@ my $psql1 = $node->background_psql('postgres');
 # masked by an index scan that would hit ReadBuffer_common from nbtree.
 $psql1->query_safe(q(CREATE TEMP TABLE foo AS SELECT 42 AS val;));
 
+# Also create an empty table, so read path go straight through the
+# extend-relation entry point.
+$psql1->query_safe(q(CREATE TEMP TABLE empty_foo (val INT);));
+
 # Resolve the owner's temp schema so the probing session can refer to
 # the table by a fully-qualified name.
 my $tempschema = $node->safe_psql(
@@ -67,6 +71,18 @@ like(
 	qr/cannot access temporary tables of other sessions/,
 	'SELECT (seqscan via read_stream)');
 
+# INSERT into empty table goes through hio.c which calls RelationAddBlocks() to
+# extend the table; that hits the check before new pages are created for the
+# table.
+$node->psql(
+	'postgres',
+	"INSERT INTO $tempschema.empty_foo VALUES (42);",
+	stderr => \$stderr);
+like(
+	$stderr,
+	qr/cannot access temporary tables of other sessions/,
+	'INSERT (caught via hio.c)');
+
 # INSERT goes through hio.c which calls ReadBufferExtended() to find a
 # page with free space; that hits the existing check before any data
 # is written.