diff --git a/docs/ref/forms/api.txt b/docs/ref/forms/api.txt
index 23fbab9aff..0c7614bba5 100644
--- a/docs/ref/forms/api.txt
+++ b/docs/ref/forms/api.txt
@@ -992,7 +992,7 @@ Methods of ``BoundField``
     ``only_initial`` is used by Django internals and should not be set
     explicitly.
 
-.. method:: BoundField.css_classes()
+.. method:: BoundField.css_classes(extra_classes=None)
 
     When you use Django's rendering shortcuts, CSS classes are used to
     indicate required form fields or fields that contain errors. If you're