diff --git a/src/borg/_hashindex.c b/src/borg/_hashindex.c
index 14206efe1..d49574bc8 100644
--- a/src/borg/_hashindex.c
+++ b/src/borg/_hashindex.c
@@ -160,6 +160,8 @@ hashindex_lookup(HashIndex *index, const void *key, int *start_idx)
         }
         else if(BUCKET_MATCHES_KEY(index, idx, key)) {
             if (didx != -1) {
+                // note: although lookup is logically a read-only operation,
+                // we optimize (change) the hashindex here "on the fly".
                 memcpy(BUCKET_ADDR(index, didx), BUCKET_ADDR(index, idx), index->bucket_size);
                 BUCKET_MARK_DELETED(index, idx);
                 idx = didx;
@@ -592,7 +594,7 @@ hashindex_delete(HashIndex *index, const void *key)
 {
     int idx = hashindex_lookup(index, key, NULL);
     if (idx < 0) {
-        return 1;
+        return -1;
     }
     BUCKET_MARK_DELETED(index, idx);
     index->num_entries -= 1;
diff --git a/src/borg/hashindex.pyx b/src/borg/hashindex.pyx
index bf84d6d49..b8e86d142 100644
--- a/src/borg/hashindex.pyx
+++ b/src/borg/hashindex.pyx
@@ -9,7 +9,7 @@ from libc.errno cimport errno
 from cpython.exc cimport PyErr_SetFromErrnoWithFilename
 from cpython.buffer cimport PyBUF_SIMPLE, PyObject_GetBuffer, PyBuffer_Release
 
-API_VERSION = '1.1_04'
+API_VERSION = '1.1_05'
 
 
 cdef extern from "_hashindex.c":
@@ -117,7 +117,12 @@ cdef class IndexBase:
 
     def __delitem__(self, key):
         assert len(key) == self.key_size
-        if not hashindex_delete(self.index, <char *>key):
+        rc = hashindex_delete(self.index, <char *>key)
+        if rc == 1:
+            return  # success
+        if rc == -1:
+            raise KeyError(key)
+        if rc == 0:
             raise Exception('hashindex_delete failed')
 
     def get(self, key, default=None):
diff --git a/src/borg/helpers.py b/src/borg/helpers.py
index fdacd1418..9100bedf5 100644
--- a/src/borg/helpers.py
+++ b/src/borg/helpers.py
@@ -131,7 +131,7 @@ class MandatoryFeatureUnsupported(Error):
 
 def check_extension_modules():
     from . import platform, compress, item
-    if hashindex.API_VERSION != '1.1_04':
+    if hashindex.API_VERSION != '1.1_05':
         raise ExtensionModuleError
     if chunker.API_VERSION != '1.1_01':
         raise ExtensionModuleError
diff --git a/src/borg/testsuite/hashindex.py b/src/borg/testsuite/hashindex.py
index 31e3d7a76..0b4b3bc5f 100644
--- a/src/borg/testsuite/hashindex.py
+++ b/src/borg/testsuite/hashindex.py
@@ -73,11 +73,11 @@ class HashIndexTestCase(BaseTestCase):
 
     def test_nsindex(self):
         self._generic_test(NSIndex, lambda x: (x, x),
-                           'b96ec1ddabb4278cc92261ee171f7efc979dc19397cc5e89b778f05fa25bf93f')
+                           '85f72b036c692c8266e4f51ccf0cff2147204282b5e316ae508d30a448d88fef')
 
     def test_chunkindex(self):
         self._generic_test(ChunkIndex, lambda x: (x, x, x),
-                           '9d437a1e145beccc790c69e66ba94fc17bd982d83a401c9c6e524609405529d8')
+                           'c83fdf33755fc37879285f2ecfc5d1f63b97577494902126b6fb6f3e4d852488')
 
     def test_resize(self):
         n = 2000  # Must be >= MIN_BUCKETS