diff --git a/cache.h b/cache.h
index 2475de9fa837596303284157e08b3080d64351ee..7b5cc834ab5df95a9589fa83e0236806b9c77097 100644
--- a/cache.h
+++ b/cache.h
@@ -126,6 +126,7 @@ struct cache_entry {
 
 #define CE_NAMEMASK  (0x0fff)
 #define CE_STAGEMASK (0x3000)
+#define CE_EXTENDED  (0x4000)
 #define CE_VALID     (0x8000)
 #define CE_STAGESHIFT 12
 
diff --git a/read-cache.c b/read-cache.c
index 2c03ec3069decb20f7557af4ac7dbe295f2dcf9c..f0ba2247980cc938cc3f821a18e3b328966c4e06 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1118,6 +1118,10 @@ static void convert_from_disk(struct ondisk_cache_entry *ondisk, struct cache_en
 	ce->ce_size  = ntohl(ondisk->size);
 	/* On-disk flags are just 16 bits */
 	ce->ce_flags = ntohs(ondisk->flags);
+
+	/* For future extension: we do not understand this entry yet */
+	if (ce->ce_flags & CE_EXTENDED)
+		die("Unknown index entry format");
 	hashcpy(ce->sha1, ondisk->sha1);
 
 	len = ce->ce_flags & CE_NAMEMASK;