diff --git a/sha1_file.c b/sha1_file.c
index d451a94efed7d36d55cf1b77324eaf55e0d50d69..6b7577dbc4ed845da1a70b6d727370423b086735 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -321,12 +321,16 @@ struct packed_git *packed_git;
 static int check_packed_git_idx(const char *path, unsigned long *idx_size_,
 				void **idx_map_)
 {
+	SHA_CTX ctx;
+	unsigned char sha1[20];
 	void *idx_map;
 	unsigned int *index;
 	unsigned long idx_size;
 	int nr, i;
-	int fd = open(path, O_RDONLY);
+	int fd;
 	struct stat st;
+
+	fd = open(path, O_RDONLY);
 	if (fd < 0)
 		return -1;
 	if (fstat(fd, &st)) {
@@ -364,6 +368,16 @@ static int check_packed_git_idx(const char *path, unsigned long *idx_size_,
 	if (idx_size != 4*256 + nr * 24 + 20 + 20)
 		return error("wrong index file size");
 
+	/*
+	 * File checksum.
+	 */
+	SHA1_Init(&ctx);
+	SHA1_Update(&ctx, idx_map, idx_size-20);
+	SHA1_Final(sha1, &ctx);
+
+	if (memcmp(sha1, idx_map + idx_size - 20, 20))
+		return error("index checksum mismatch");
+
 	return 0;
 }
 
diff --git a/t/t5300-pack-object.sh b/t/t5300-pack-object.sh
index 5b50536b54ff84b6a6e51c210be39773c6879443..7dfb1ab77b070447c4dd2b9f645785feec6f5480 100755
--- a/t/t5300-pack-object.sh
+++ b/t/t5300-pack-object.sh
@@ -163,6 +163,13 @@ test_expect_success \
      else :;
      fi &&
 
+     cp test-1-${packname_1}.pack test-3.pack &&
+     dd if=/dev/zero of=test-3.idx count=1 bs=1 conv=notrunc seek=1200 &&
+     if git-verify-pack test-3.pack
+     then false
+     else :;
+     fi &&
+
      :'
 
 test_expect_success \