diff --git a/content/client-server-api/_index.md b/content/client-server-api/_index.md
index 1a6f1c6e..1311c33a 100644
--- a/content/client-server-api/_index.md
+++ b/content/client-server-api/_index.md
@@ -218,6 +218,9 @@ only read state (e.g.: `/sync`, get account data, etc).
 The user is unable to reject an invite to join the server notices room.
 See the [Server Notices](#server-notices) module for more information.
 
+`M_THREEPID_MEDIUM_NOT_SUPPORTED`
+The homeserver does not support adding a third party identifier of the given medium.
+
 #### Rate limiting
 
 Homeservers SHOULD implement rate limiting to reduce the risk of being
diff --git a/data/api/client-server/registration.yaml b/data/api/client-server/registration.yaml
index 84aef5b1..316506af 100644
--- a/data/api/client-server/registration.yaml
+++ b/data/api/client-server/registration.yaml
@@ -284,6 +284,10 @@ paths:
               has an account on the homeserver in question.
             * `M_SERVER_NOT_TRUSTED` : The `id_server` parameter refers to an identity server
               that is not trusted by this homeserver.
+            * `M_THREEPID_MEDIUM_NOT_SUPPORTED`
+              The homeserver does not support adding email addresses.
+            * `M_INVALID_PARAM`
+              The email address given was not valid.
           content:
             application/json:
               schema:
@@ -345,6 +349,10 @@ paths:
               has an account on the homeserver in question.
             * `M_SERVER_NOT_TRUSTED` : The `id_server` parameter refers to an identity server
               that is not trusted by this homeserver.
+            * `M_THREEPID_MEDIUM_NOT_SUPPORTED`
+              The homeserver does not support adding phone numbers.
+            * `M_INVALID_PARAM`
+              The phone number given was not valid.
           content:
             application/json:
               schema: