diff --git a/builtin/merge.c b/builtin/merge.c
index fb3eb15..888166d 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -703,7 +703,7 @@ static int try_merge_strategy(const char *strategy, struct commit_list *common,
 	if (repo_refresh_and_write_index(the_repository, REFRESH_QUIET,
 					 SKIP_IF_UNCHANGED, 0, NULL, NULL,
 					 NULL) < 0)
-		return error(_("Unable to write index."));
+		die(_("Unable to write index."));
 
 	if (!strcmp(strategy, "recursive") || !strcmp(strategy, "subtree") ||
 	    !strcmp(strategy, "ort")) {
diff --git a/t/t7600-merge.sh b/t/t7600-merge.sh
index e5ff073..ef54cff 100755
--- a/t/t7600-merge.sh
+++ b/t/t7600-merge.sh
@@ -236,6 +236,16 @@
 	verify_parents $c1 $c2
 '
 
+test_expect_success 'merge c1 with c2 when index.lock exists' '
+	test_when_finished rm .git/index.lock &&
+	git reset --hard c1 &&
+	>.git/index.lock &&
+	test_must_fail git merge c2 &&
+	test_path_is_missing .git/MERGE_HEAD &&
+	test_path_is_missing .git/MERGE_MODE &&
+	test_path_is_missing .git/MERGE_MSG
+'
+
 test_expect_success 'merge --squash c3 with c7' '
 	git reset --hard c3 &&
 	test_must_fail git merge --squash c7 &&