Adding a new context to a custom field produces "[webwork.view.taglib.IteratorTag] Value is null! Returning an empty set"

Trying to add a new context to a custom field produces an entry in the log file of the form [webwork.view.taglib.IteratorTag] Value is null! Returning an empty set. This normally indicates that some form of Java exception is being thrown and eventually swallowed by a webwork tag.

This does not appear to affect the page in any visual way. It does not stop the user from adding a context.