diff --git a/api/client-server/registration.yaml b/api/client-server/registration.yaml
index fe57f950..3818fc34 100644
--- a/api/client-server/registration.yaml
+++ b/api/client-server/registration.yaml
@@ -100,6 +100,7 @@ paths:
             Part of the request was invalid. This may include one of the following error codes:
 
             * ``M_USER_IN_USE`` : The desired user ID is already taken.
+            * ``M_INVALID_USERNAME`` : The desired user ID is not a valid user name.
             * ``M_EXCLUSIVE`` : The desired user ID is in the exclusive namespace
               claimed by an application service.
 
diff --git a/specification/client_server_api.rst b/specification/client_server_api.rst
index c9e6402c..1cd2a49d 100644
--- a/specification/client_server_api.rst
+++ b/specification/client_server_api.rst
@@ -86,6 +86,9 @@ Some requests have unique error codes:
 :``M_USER_IN_USE``:
   Encountered when trying to register a user ID which has been taken.
 
+:``M_INVALID_USERNAME``:
+  Encountered when trying to register a user ID which is not valid.
+
 :``M_ROOM_IN_USE``:
   Encountered when trying to create a room which has been taken.