diff --git a/openapi_extensions.md b/openapi_extensions.md
index 6905d663..22d1ac66 100644
--- a/openapi_extensions.md
+++ b/openapi_extensions.md
@@ -54,6 +54,8 @@ doesn't work, as in the following example:
           properties:
             ...
 ```
+This can only be used to define the type of named properties. In particular,
+the current tooling does not support `oneOf` inside `additionalProperties`.
 
 ## OpenAPI 3's "2xx" format for response codes