diff --git a/synapse/handlers/register.py b/synapse/handlers/register.py
index cda4a8502a..c25e321099 100644
--- a/synapse/handlers/register.py
+++ b/synapse/handlers/register.py
@@ -31,6 +31,7 @@ import base64
 import bcrypt
 import json
 import logging
+import urllib
 
 logger = logging.getLogger(__name__)
 
@@ -63,6 +64,13 @@ class RegistrationHandler(BaseHandler):
             password_hash = bcrypt.hashpw(password, bcrypt.gensalt())
 
         if localpart:
+            if localpart and urllib.quote(localpart) != localpart:
+                raise SynapseError(
+                    400,
+                    "User ID must only contain characters which do not"
+                    " require URL encoding."
+                )
+
             user = UserID(localpart, self.hs.hostname)
             user_id = user.to_string()
 
diff --git a/synapse/rest/client/v1/register.py b/synapse/rest/client/v1/register.py
index ccc457924b..a56834e365 100644
--- a/synapse/rest/client/v1/register.py
+++ b/synapse/rest/client/v1/register.py
@@ -27,7 +27,6 @@ from hashlib import sha1
 import hmac
 import simplejson as json
 import logging
-import urllib
 
 logger = logging.getLogger(__name__)
 
@@ -263,14 +262,11 @@ class RegisterRestServlet(ClientV1RestServlet):
             )
 
         password = register_json["password"].encode("utf-8")
-        desired_user_id = (register_json["user"].encode("utf-8")
-                           if "user" in register_json else None)
-        if (desired_user_id
-                and urllib.quote(desired_user_id) != desired_user_id):
-            raise SynapseError(
-                400,
-                "User ID must only contain characters which do not " +
-                "require URL encoding.")
+        desired_user_id = (
+            register_json["user"].encode("utf-8")
+            if "user" in register_json else None
+        )
+
         handler = self.handlers.registration_handler
         (user_id, token) = yield handler.register(
             localpart=desired_user_id,