diff --git a/data/api/client-server/receipts.yaml b/data/api/client-server/receipts.yaml
index e795ce60..7a8dc279 100644
--- a/data/api/client-server/receipts.yaml
+++ b/data/api/client-server/receipts.yaml
@@ -100,9 +100,11 @@ paths:
             "$ref": "definitions/errors/rate_limited.yaml"
         400:
           description: |-
-            The `thread_id` is not a string, or is empty, or it is provided for
-            a receipt of type `m.fully_read`, or the `event_id` is not related
-            to the `thread_id`.
+            The `thread_id` is invalid in some way. For example:
+            * It is not a string.
+            * It is empty.
+            * It is provided for an incompatible receipt type.
+            * The `event_id` is not related to the `thread_id`.
           schema:
             $ref: "definitions/errors/error.yaml"
           examples: