diff --git a/src/borg/hashindex.pyx b/src/borg/hashindex.pyx
index 916df65ed..9ce87abcd 100644
--- a/src/borg/hashindex.pyx
+++ b/src/borg/hashindex.pyx
@@ -205,7 +205,7 @@ cdef class NSIndex(IndexBase):
     def __getitem__(self, key):
         assert len(key) == self.key_size
         data = <uint32_t *>hashindex_get(self.index, <unsigned char *>key)
-        if not data:
+        if data == NULL:
             raise KeyError(key)
         cdef uint32_t segment = _le32toh(data[0])
         assert segment <= _MAX_VALUE, "maximum number of segments reached"
@@ -237,8 +237,8 @@ cdef class NSIndex(IndexBase):
         iter.index = self.index
         if marker:
             key = hashindex_get(self.index, <unsigned char *>marker)
-            if marker is None:
-                raise IndexError
+            if key == NULL:
+                raise KeyError("marker not found")
             iter.key = key - self.key_size
         return iter
 
@@ -296,7 +296,7 @@ cdef class ChunkIndex(IndexBase):
     def __getitem__(self, key):
         assert len(key) == self.key_size
         data = <uint32_t *>hashindex_get(self.index, <unsigned char *>key)
-        if not data:
+        if data == NULL:
             raise KeyError(key)
         cdef uint32_t refcount = _le32toh(data[0])
         assert refcount <= _MAX_VALUE, "invalid reference count"
@@ -324,7 +324,7 @@ cdef class ChunkIndex(IndexBase):
         """Increase refcount for 'key', return (refcount, size, csize)."""
         assert len(key) == self.key_size
         data = <uint32_t *>hashindex_get(self.index, <unsigned char *>key)
-        if not data:
+        if data == NULL:
             raise KeyError(key)
         cdef uint32_t refcount = _le32toh(data[0])
         assert refcount <= _MAX_VALUE, "invalid reference count"
@@ -337,7 +337,7 @@ cdef class ChunkIndex(IndexBase):
         """Decrease refcount for 'key', return (refcount, size, csize)."""
         assert len(key) == self.key_size
         data = <uint32_t *>hashindex_get(self.index, <unsigned char *>key)
-        if not data:
+        if data == NULL:
             raise KeyError(key)
         cdef uint32_t refcount = _le32toh(data[0])
         # Never decrease a reference count of zero
@@ -354,8 +354,8 @@ cdef class ChunkIndex(IndexBase):
         iter.index = self.index
         if marker:
             key = hashindex_get(self.index, <unsigned char *>marker)
-            if marker is None:
-                raise IndexError
+            if key == NULL:
+                raise KeyError("marker not found")
             iter.key = key - self.key_size
         return iter
 
@@ -406,7 +406,7 @@ cdef class ChunkIndex(IndexBase):
                 break
             our_values = <const uint32_t*> (key + self.key_size)
             master_values = <const uint32_t*> hashindex_get(master, key)
-            if not master_values:
+            if master_values == NULL:
                 raise ValueError('stats_against: key contained in self but not in master_index.')
             our_refcount = _le32toh(our_values[0])
             chunk_size = _le32toh(master_values[1])
@@ -434,7 +434,7 @@ cdef class ChunkIndex(IndexBase):
     cdef _add(self, unsigned char *key, uint32_t *data):
         cdef uint64_t refcount1, refcount2, result64
         values = <uint32_t*> hashindex_get(self.index, key)
-        if values:
+        if values != NULL:
             refcount1 = _le32toh(values[0])
             refcount2 = _le32toh(data[0])
             assert refcount1 <= _MAX_VALUE, "invalid reference count"
diff --git a/src/borg/testsuite/hashindex_stress.py b/src/borg/testsuite/hashindex_pytest.py
similarity index 55%
rename from src/borg/testsuite/hashindex_stress.py
rename to src/borg/testsuite/hashindex_pytest.py
index b0a819863..e81d3242c 100644
--- a/src/borg/testsuite/hashindex_stress.py
+++ b/src/borg/testsuite/hashindex_pytest.py
@@ -3,7 +3,37 @@ import random
 
 import pytest
 
-from ..hashindex import NSIndex
+from borg.hashindex import NSIndex, ChunkIndex
+
+
+def test_nsindex_iteritems_marker():
+    nsindex = NSIndex()
+    nsindex[b'\xbb'*32] = (123, 456)
+    nsindex[b'\xaa'*32] = (234, 567)
+
+    # marker exists
+    items = list(nsindex.iteritems(marker=b'\xbb'*32))
+    assert len(items) == 1
+    assert items[0][0] == b'\xaa'*32
+
+    # marker does not exist
+    with pytest.raises(KeyError, match="marker not found"):
+        list(nsindex.iteritems(marker=b'\xcc'*32))
+
+
+def test_chunkindex_iteritems_marker():
+    chunkindex = ChunkIndex()
+    chunkindex[b'\xbb'*32] = (1, 100, 50)
+    chunkindex[b'\xaa'*32] = (1, 200, 100)
+
+    # marker exists
+    items = list(chunkindex.iteritems(marker=b'\xbb'*32))
+    assert len(items) == 1
+    assert items[0][0] == b'\xaa'*32
+
+    # marker does not exist
+    with pytest.raises(KeyError, match="marker not found"):
+        list(chunkindex.iteritems(marker=b'\xcc'*32))
 
 
 @pytest.mark.skipif("BORG_TESTS_SLOW" not in os.environ, reason="slow tests not enabled, use BORG_TESTS_SLOW=1")