Why can't icefaces give proper error messages?

SEVERE: StandardWrapperValve[Persistent Faces Servlet]: PWC1406: Servlet.service() for servlet Persistent Faces Servlet threw exception
java.lang.Exception: javax.faces.FacesException: Problem in renderResponse: null

was trying to tell me:

<ice:tree> needed to be encapsulated inside an <ice:form>.

http://seamframework.org/Community/ProblemWithIceTree#comment101040