diff --git a/diff-delta.c b/diff-delta.c
index e49643353b..77fea08dfb 100644
--- a/diff-delta.c
+++ b/diff-delta.c
@@ -326,6 +326,8 @@ create_delta(const struct delta_index *index,
 	const unsigned char *ref_data, *ref_top, *data, *top;
 	unsigned char *out;
 
+	*delta_size = 0;
+
 	if (!trg_buf || !trg_size)
 		return NULL;