diff --git a/taiga/auth/serializers.py b/taiga/auth/serializers.py
index 5224b6d3..2073e000 100644
--- a/taiga/auth/serializers.py
+++ b/taiga/auth/serializers.py
@@ -50,6 +50,6 @@ class PrivateRegisterForNewUserSerializer(BaseRegisterSerializer):
 
 
 class PrivateRegisterForExistingUserSerializer(serializers.Serializer):
-    username = serializers.CharField(max_length=30)
+    username = serializers.CharField(max_length=255)
     password = serializers.CharField(min_length=4)
     token = serializers.CharField(max_length=255, required=True)