diff --git a/src/borg/_hashindex.c b/src/borg/_hashindex.c
index 636b85b99..cee34269a 100644
--- a/src/borg/_hashindex.c
+++ b/src/borg/_hashindex.c
@@ -716,10 +716,10 @@ hashindex_len(HashIndex *index)
     return index->num_entries;
 }
 
-static int
+static int64_t
 hashindex_size(HashIndex *index)
 {
-    return sizeof(HashHeader) + index->num_buckets * index->bucket_size;
+    return sizeof(HashHeader) + (int64_t)index->num_buckets * index->bucket_size;
 }
 
 /*
diff --git a/src/borg/hashindex.pyx b/src/borg/hashindex.pyx
index 28f60ef9c..26f78faa6 100644
--- a/src/borg/hashindex.pyx
+++ b/src/borg/hashindex.pyx
@@ -5,7 +5,7 @@ import locale
 import os
 
 cimport cython
-from libc.stdint cimport uint32_t, UINT32_MAX, uint64_t
+from libc.stdint cimport int64_t, uint32_t, UINT32_MAX, uint64_t
 from libc.errno cimport errno
 from libc.string cimport memcpy
 from cpython.exc cimport PyErr_SetFromErrnoWithFilename
@@ -28,7 +28,7 @@ cdef extern from "_hashindex.c":
     HashIndex *hashindex_init(int capacity, int key_size, int value_size)
     void hashindex_free(HashIndex *index)
     int hashindex_len(HashIndex *index)
-    int hashindex_size(HashIndex *index)
+    int64_t hashindex_size(HashIndex *index)
     void hashindex_write(HashIndex *index, object file_py) except *
     void *hashindex_get(HashIndex *index, void *key)
     void *hashindex_next_key(HashIndex *index, void *key)