diff --git a/src/Application/Model/Configuration.php b/src/Application/Model/Configuration.php
index 90f8777..6bdd750 100644
--- a/src/Application/Model/Configuration.php
+++ b/src/Application/Model/Configuration.php
@@ -58,7 +58,7 @@ class Configuration
         $country = trim(Registry::getConfig()->getConfigParam('d3linkmobility_smsSenderCountry'));
         $country = strlen($country) ? strtoupper($country) : null;
 
-        Assert::that($country)->null() || Assert::that($country)->string()->length(2);
+        Assert::that($country)->nullOr() || Assert::that($country)->string()->length(2);
 
         return $country;
     }