diff --git a/read-cache.c b/read-cache.c
index 309b77a6c..f1f789b7b 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -26,7 +26,7 @@ static struct cache_entry *refresh_cache_entry(struct cache_entry *ce, int reall
 
 #define CACHE_EXT(s) ( (s[0]<<24)|(s[1]<<16)|(s[2]<<8)|(s[3]) )
 #define CACHE_EXT_TREE 0x54524545	/* "TREE" */
-#define CACHE_EXT_RESOLVE_UNDO 0x52455543 /* "REUN" */
+#define CACHE_EXT_RESOLVE_UNDO 0x52455543 /* "REUC" */
 
 struct index_state the_index;