diff --git a/src/Common/FormData.php b/src/Common/FormData.php
index e21fbe2..8041257 100644
--- a/src/Common/FormData.php
+++ b/src/Common/FormData.php
@@ -65,7 +65,7 @@ abstract class FormData
     {
         $username = self::validateMailAddress($username, $required, 'Username');
 
-        if (strpos($username, '%') !== false) {
+        if ($username !== null && strpos($username, '%') !== false) {
             throw new InputValidationError('Username must not contain the wildcard character "%" (use a wildcard alias instead).');
         }
         return $username;