diff --git a/synapse/handlers/register.py b/synapse/handlers/register.py
index bd5ba6e1d8..f6e7e58563 100644
--- a/synapse/handlers/register.py
+++ b/synapse/handlers/register.py
@@ -15,7 +15,6 @@
 
 """Contains functions for registering clients."""
 import logging
-import urllib
 
 from twisted.internet import defer
 
@@ -23,6 +22,7 @@ from synapse.api.errors import (
     AuthError, Codes, SynapseError, RegistrationError, InvalidCaptchaError
 )
 from synapse.http.client import CaptchaServerHttpClient
+from synapse import types
 from synapse.types import UserID
 from synapse.util.async import run_on_reactor
 from ._base import BaseHandler
@@ -47,7 +47,7 @@ class RegistrationHandler(BaseHandler):
     @defer.inlineCallbacks
     def check_username(self, localpart, guest_access_token=None,
                        assigned_user_id=None):
-        if urllib.quote(localpart.encode('utf-8')) != localpart:
+        if types.contains_invalid_mxid_characters(localpart):
             raise SynapseError(
                 400,
                 "User ID can only contain characters a-z, 0-9, or '=_-./'",
@@ -253,7 +253,7 @@ class RegistrationHandler(BaseHandler):
         """
         Registers email_id as SAML2 Based Auth.
         """
-        if urllib.quote(localpart) != localpart:
+        if types.contains_invalid_mxid_characters(localpart):
             raise SynapseError(
                 400,
                 "User ID can only contain characters a-z, 0-9, or '=_-./'",