diff --git a/builtin/grep.c b/builtin/grep.c
index 0e1b6c860e..8025964987 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -823,6 +823,8 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 			struct object *object = parse_object(sha1);
 			if (!object)
 				die(_("bad object %s"), arg);
+			if (!seen_dashdash)
+				verify_non_filename(prefix, arg);
 			add_object_array(object, arg, &list);
 			continue;
 		}
diff --git a/setup.c b/setup.c
index 1ccfafaa7a..2e1521b09e 100644
--- a/setup.c
+++ b/setup.c
@@ -66,7 +66,14 @@ int check_filename(const char *prefix, const char *arg)
 	const char *name;
 	struct stat st;
 
-	name = prefix ? prefix_filename(prefix, strlen(prefix), arg) : arg;
+	if (!prefixcmp(arg, ":/")) {
+		if (arg[2] == '\0') /* ":/" is root dir, always exists */
+			return 1;
+		name = arg + 2;
+	} else if (prefix)
+		name = prefix_filename(prefix, strlen(prefix), arg);
+	else
+		name = arg;
 	if (!lstat(name, &st))
 		return 1; /* file exists */
 	if (errno == ENOENT || errno == ENOTDIR)
diff --git a/t/t4208-log-magic-pathspec.sh b/t/t4208-log-magic-pathspec.sh
index 2c482b622b..72300b5f24 100755
--- a/t/t4208-log-magic-pathspec.sh
+++ b/t/t4208-log-magic-pathspec.sh
@@ -11,11 +11,24 @@ test_expect_success 'setup' '
 	mkdir sub
 '
 
-test_expect_success '"git log :/" should be ambiguous' '
-	test_must_fail git log :/ 2>error &&
+test_expect_success '"git log :/" should not be ambiguous' '
+	git log :/
+'
+
+test_expect_success '"git log :/a" should be ambiguous (applied both rev and worktree)' '
+	: >a &&
+	test_must_fail git log :/a 2>error &&
 	grep ambiguous error
 '
 
+test_expect_success '"git log :/a -- " should not be ambiguous' '
+	git log :/a --
+'
+
+test_expect_success '"git log -- :/a" should not be ambiguous' '
+	git log -- :/a
+'
+
 test_expect_success '"git log :" should be ambiguous' '
 	test_must_fail git log : 2>error &&
 	grep ambiguous error