diff --git a/specification/3-application_service_api.rst b/specification/3-application_service_api.rst
index 557e16aa..8d0efdef 100644
--- a/specification/3-application_service_api.rst
+++ b/specification/3-application_service_api.rst
@@ -57,7 +57,7 @@ An example HS configuration required to pass traffic to the AS is:
 .. WARNING::
   If the homeserver in question has multiple application services, each
   ``as_token`` MUST be unique per application service as this token is used to
-  identify the application service.
+  identify the application service. The homeserver MUST enforce this.
 
 - An application service can state whether they should be the only ones who 
   can manage a specified namespace. This is referred to as an "exclusive"