diff --git a/api-ref/source/versions.inc b/api-ref/source/versions.inc
index 9fbecf631..552a03651 100644
--- a/api-ref/source/versions.inc
+++ b/api-ref/source/versions.inc
@@ -8,6 +8,9 @@ Concepts
 The Senlin API supports a ''major versions'' expressed in request URLs and
 ''microversions'' which can be sent in HTTP header ``OpenStack-API-Version``.
 
+When the specified ``OpenStack-API-Version`` is not supported by the API
+service, a 406 (NotAcceptable) exception will be raised. Note that this applies
+to all API requests documented in this guide.
 
 List Major Versions
 ===================