diff --git a/delta.h b/delta.h
index eb5c6d2fdb..ab0279168c 100644
--- a/delta.h
+++ b/delta.h
@@ -28,7 +28,7 @@ void free_delta_index(struct delta_index *index);
  *
  * Given pointer must be what create_delta_index() returned, or NULL.
  */
-unsigned long sizeof_delta_index(struct delta_index *index);
+size_t sizeof_delta_index(struct delta_index *index);
 
 /*
  * create_delta: create a delta from given index for the given buffer
diff --git a/diff-delta.c b/diff-delta.c
index 43c339f010..e07e6a90a1 100644
--- a/diff-delta.c
+++ b/diff-delta.c
@@ -302,7 +302,7 @@ void free_delta_index(struct delta_index *index)
 	free(index);
 }
 
-unsigned long sizeof_delta_index(struct delta_index *index)
+size_t sizeof_delta_index(struct delta_index *index)
 {
 	if (index)
 		return index->memsize;