#7311 choice_value callback argument can be null

Author: Michael Lenahan

reference/forms/types/options/choice_value.rst.inc

@@ -14,6 +14,9 @@ configure the value that will be sent in the API request).
 This can be a callable or a property path. See `choice_label`_ for similar usage.
 If ``null`` is used, an incrementing integer is used as the name.
 
+If you are using a callable to populate choice_value, you need to check for the
+case that the value of the field may be null.
+
 .. caution::
 
     In Symfony 2.7, there was a small backwards-compatibility break with how the