diff --git a/delta.h b/delta.h
index ab0279168c..12075c54c5 100644
--- a/delta.h
+++ b/delta.h
@@ -14,7 +14,7 @@ struct delta_index;
  * using free_delta_index().
  */
 struct delta_index *
-create_delta_index(const void *buf, unsigned long bufsize);
+create_delta_index(const void *buf, size_t bufsize);
 
 /*
  * free_delta_index: free the index created by create_delta_index()
diff --git a/diff-delta.c b/diff-delta.c
index 9e1f9e6f95..bcc331af3e 100644
--- a/diff-delta.c
+++ b/diff-delta.c
@@ -132,7 +132,7 @@ struct delta_index {
 	struct index_entry *hash[FLEX_ARRAY];
 };
 
-struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)
+struct delta_index * create_delta_index(const void *buf, size_t bufsize)
 {
 	unsigned int i, hsize, hmask, entries, prev_val, *hash_count;
 	const unsigned char *data, *buffer = buf;