diff --git a/api-ref/source/parameters.yaml b/api-ref/source/parameters.yaml
index a2d93c5ef02e..36bc57e7a337 100644
--- a/api-ref/source/parameters.yaml
+++ b/api-ref/source/parameters.yaml
@@ -3453,9 +3453,14 @@ ip_range_delete:
 key_name:
   description: |
     Key pair name.
+    .. note::
+
+      The ``null`` value allowed in Nova legacy v2 API, but due to the strict
+      input validation, it isn't allowed in Nova v2.1 API.
   in: body
   required: false
   type: string
+
 key_name_resp:
   description: |
     The name of associated key pair, if any.