diff --git a/builtin/merge-file.c b/builtin/merge-file.c
index 084fdfec58..59a9792208 100644
--- a/builtin/merge-file.c
+++ b/builtin/merge-file.c
@@ -108,7 +108,8 @@ int cmd_merge_file(int argc,
 			return error_errno("failed to redirect stderr to /dev/null");
 	}
 
-	if (object_id)
+	if (!repo && object_id)
+		/* emit the correct "not a git repo" error in this case */
 		setup_git_directory();
 
 	for (i = 0; i < 3; i++) {
diff --git a/t/t6403-merge-file.sh b/t/t6403-merge-file.sh
index 4d6e748320..801284cf8f 100755
--- a/t/t6403-merge-file.sh
+++ b/t/t6403-merge-file.sh
@@ -542,6 +542,15 @@ test_expect_success '--object-id fails without repository' '
 	grep "not a git repository" err
 '
 
+test_expect_success 'run in a linked worktree with --object-id' '
+	empty="$(test_oid empty_blob)" &&
+	git worktree add work &&
+	git -C work merge-file --object-id $empty $empty $empty >actual &&
+	git worktree remove work &&
+	git merge-file --object-id $empty $empty $empty >expected &&
+	test_cmp actual expected
+'
+
 test_expect_success 'merging C files with "myers" diff algorithm creates some spurious conflicts' '
 	cat >expect.c <<-\EOF &&
 	int g(size_t u)