diff --git a/php/utils/tags.php b/php/utils/tags.php
index 190a92d..446259c 100644
--- a/php/utils/tags.php
+++ b/php/utils/tags.php
@@ -48,7 +48,7 @@
          */
         public static function cacheable_room_tags(array $tags) {
             return array_map(
-                'CommunityTag::__toString',
+                function (\CommunityTag $tag) { return $tag->__toString(); },
                 array_filter($tags, function(\CommunityTag $tag) {
                     return $tag->type == TagType::USER_TAG;
                 })