diff --git a/firstuseauthenticator/firstuseauthenticator.py b/firstuseauthenticator/firstuseauthenticator.py
index 1b45aa5..7062b19 100644
--- a/firstuseauthenticator/firstuseauthenticator.py
+++ b/firstuseauthenticator/firstuseauthenticator.py
@@ -138,7 +138,7 @@ def validate_username(self, name):
 
     @gen.coroutine
     def authenticate(self, handler, data):
-        username = data['username'].lower()
+        username = self.normalize_username(data['username'])
 
         if not self.create_users:
             if not self._user_exists(username):