diff --git a/data/api/client-server/receipts.yaml b/data/api/client-server/receipts.yaml
index d22edb1e..e795ce60 100644
--- a/data/api/client-server/receipts.yaml
+++ b/data/api/client-server/receipts.yaml
@@ -100,7 +100,7 @@ paths:
             "$ref": "definitions/errors/rate_limited.yaml"
         400:
           description: |-
-            The `thread_id is not a string, or is empty, or it is provided for
+            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`.
           schema: