diff --git a/diff-no-index.c b/diff-no-index.c
index aae8e7accc1ff9..4cd9dacbe8e98d 100644
--- a/diff-no-index.c
+++ b/diff-no-index.c
@@ -150,16 +150,14 @@ static int queue_diff(struct diff_options *o,
 
 static int path_outside_repo(const char *path)
 {
-	/*
-	 * We have already done setup_git_directory_gently() so we
-	 * know we are inside a git work tree already.
-	 */
 	const char *work_tree;
 	size_t len;
 
 	if (!is_absolute_path(path))
 		return 0;
 	work_tree = get_git_work_tree();
+	if (!work_tree)
+		return 1;
 	len = strlen(work_tree);
 	if (strncmp(path, work_tree, len) ||
 	    (path[len] != '\0' && path[len] != '/'))