diff --git a/cache-tree.c b/cache-tree.c
index c53f7de2b1..648e187491 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -299,6 +299,8 @@ static int update_one(struct cache_tree *it,
 				    flags);
 		if (subcnt < 0)
 			return subcnt;
+		if (!subcnt)
+			die("index cache-tree records empty sub-tree");
 		i += subcnt;
 		sub->count = subcnt; /* to be used in the next loop */
 		*skip_count += subskip;