diff --git a/lib/Server/Url.php b/lib/Server/Url.php
index bcfabd935..fead13fbb 100644
--- a/lib/Server/Url.php
+++ b/lib/Server/Url.php
@@ -49,7 +49,7 @@ public function AddQueryString($name, $value)
         }
 
         $this->hasQuestionMark = true;
-        $this->url .= sprintf("$char%s=%s", $name, urlencode($value));
+        $this->url .= sprintf("$char%s=%s", $name, urlencode($value ?? ""));
 
         return $this;
     }