diff --git a/diff-delta.c b/diff-delta.c
index 25a798d050..74486b1b81 100644
--- a/diff-delta.c
+++ b/diff-delta.c
@@ -22,6 +22,7 @@
 #include <string.h>
 #include "delta.h"
 
+#include "git-compat-util.h"
 
 /* maximum hash entry list for the same hash bucket */
 #define HASH_LIMIT 64
@@ -131,7 +132,7 @@ struct delta_index {
 	const void *src_buf;
 	unsigned long src_size;
 	unsigned int hash_mask;
-	struct index_entry *hash[0];
+	struct index_entry *hash[FLEX_ARRAY];
 };
 
 struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)