[Zope] Customising the calling of the _add method for a product

I have a TimeSheet product which is currently utilising the suggestions from one of the howtos on zope.org, to make the handling of errors a little smarter. Subsequently my addForm code checks to see if any errors were detected and, if not, tries to go to the usual _add method. But this fails. The code below shows what I currently have. If I can just call this _add method I am finished. <dtml-if "I_Error==1"> <dtml-var TimeSheet_addForm_include> <dtml-else> <dtml-call "TimeSheet_add(_,_.None)"> </dtml-if> As always, any help greatfully received Ian