diff --git a/refs.c b/refs.c
index b9cf284f31..d5bfd11790 100644
--- a/refs.c
+++ b/refs.c
@@ -3702,6 +3702,8 @@ int ref_transaction_delete(struct ref_transaction *transaction,
 			   unsigned int flags, const char *msg,
 			   struct strbuf *err)
 {
+	if (old_sha1 && is_null_sha1(old_sha1))
+		die("BUG: delete called with old_sha1 set to zeros");
 	return ref_transaction_update(transaction, refname,
 				      null_sha1, old_sha1,
 				      flags, msg, err);