diff --git a/src/Utility/LinkitHelper.php b/src/Utility/LinkitHelper.php
index fd9b75f..a039450 100644
--- a/src/Utility/LinkitHelper.php
+++ b/src/Utility/LinkitHelper.php
@@ -92,7 +92,7 @@ class LinkitHelper {
 
     $protocol_matches = [];
     preg_match('/^([a-z]*?):/', $input, $protocol_matches);
-    if (strpos($input, "/$public_files_dir") === 0) {
+    if (!empty($public_files_dir) && strpos($input, "/$public_files_dir") === 0) {
       return "base:$input";
     }
     elseif (count($protocol_matches) > 1 && in_array($protocol_matches[1], UrlHelper::getAllowedProtocols())) {