diff --git a/src/OpenID/AbstractProvider.php b/src/OpenID/AbstractProvider.php
index 0eef9032..10a7fd40 100644
--- a/src/OpenID/AbstractProvider.php
+++ b/src/OpenID/AbstractProvider.php
@@ -100,7 +100,7 @@ protected function getRequiredRequestParameter(array $requestParameters, string
             return $requestParameters[$key];
         }
 
-        throw new Unauthorized("There is no required parameter called: '${key}'");
+        throw new Unauthorized("There is no required parameter called: '{$key}'");
     }
 
     /**