diff --git a/lib/outputrequirementslib.php b/lib/outputrequirementslib.php
index e6e02ba8292..3156a3e45dd 100644
--- a/lib/outputrequirementslib.php
+++ b/lib/outputrequirementslib.php
@@ -722,7 +722,7 @@ class page_requirements_manager {
             }
             // The URL is not a Moodle resource.
             return $url;
-        } else if (strpos($url, '/') === 0) {
+        } else if (null !== $url && strpos($url, '/') === 0) {
             // Fix the admin links if needed.
             if ($CFG->admin !== 'admin') {
                 if (strpos($url, "/admin/") === 0) {