[Zope] Error handling and Zpublisher type casts ?

Hallo ! I would like to ask how I can catch an error produced by a Zpublisher type cast. If I have an zpt with a html form with input box <input type="text" name="number:int"> then the Zpublisher tries to convert the string I typed in into an integer number. If this fails, e.g. if I type in an string "aaa" that cannot be converted instead of a number "123", then I get this error: Zope Error Zope has encountered an error while publishing this resource. Error Type: ValueError Error Value: An integer was expected in the value 'aaa' How can I catch this error ? Thank you Andreas Martin