diff --git a/builtin/merge.c b/builtin/merge.c
index 41467e4277..eb9415119f 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -822,6 +822,14 @@ static int merge_trivial(struct commit *head, struct commit_list *remoteheads)
 {
 	unsigned char result_tree[20], result_commit[20];
 	struct commit_list *parents, **pptr = &parents;
+	static struct lock_file lock;
+
+	hold_locked_index(&lock, 1);
+	refresh_cache(REFRESH_QUIET);
+	if (active_cache_changed &&
+	    write_locked_index(&the_index, &lock, COMMIT_LOCK))
+		return error(_("Unable to write index."));
+	rollback_lock_file(&lock);
 
 	write_tree_trivial(result_tree);
 	printf(_("Wonderful.\n"));
diff --git a/t/t7605-merge-resolve.sh b/t/t7605-merge-resolve.sh
index 2f80037c9a..5d56c38546 100755
--- a/t/t7605-merge-resolve.sh
+++ b/t/t7605-merge-resolve.sh
@@ -43,7 +43,7 @@ merge_c1_to_c2_cmds='
 
 test_expect_success 'merge c1 to c2'        "$merge_c1_to_c2_cmds"
 
-test_expect_failure 'merge c1 to c2, again' "$merge_c1_to_c2_cmds"
+test_expect_success 'merge c1 to c2, again' "$merge_c1_to_c2_cmds"
 
 test_expect_success 'merge c2 to c3 (fails)' '
 	git reset --hard c2 &&