diff --git a/lib/filebrowser/file_info.php b/lib/filebrowser/file_info.php
index cba268f39b7..2127d1f4f60 100644
--- a/lib/filebrowser/file_info.php
+++ b/lib/filebrowser/file_info.php
@@ -100,8 +100,8 @@ abstract class file_info {
      */
     protected function build_search_files_sql($extensions, $prefix = null) {
         global $DB;
-        if (strlen($prefix ?? '')) {
-            $prefix = $prefix.'.';
+        if ($prefix && strlen($prefix)) {
+            $prefix = $prefix . '.';
         } else {
             $prefix = '';
         }