diff --git a/src/Url.php b/src/Url.php
index f886989..eff949a 100644
--- a/src/Url.php
+++ b/src/Url.php
@@ -538,17 +538,21 @@ class Url {
    *
    * @param string $url The URL
    * @param string $parameter The parameter name
-   * @param string $value The parameter value
+   * @param string|null $value The parameter value (optional)
    * @param boolean $encode Set if parameter value must be URL encoded (optional, default: true)
    *
    * @return string The completed URL
    */
-  public static function add_url_parameter($url, $parameter, $value, $encode=true) {
+  public static function add_url_parameter($url, $parameter, $value=null, $encode=true) {
     if (strpos($url, '?') === false)
       $url .= '?';
     else
       $url .= '&';
-    $url .= "$parameter=".($encode?urlencode($value):$value);
+    $url .= (
+      is_null($value)?
+      $parameter:
+      "$parameter=".($encode?urlencode($value):$value)
+    );
     return $url;
   }