[Zope] Passing an instance in a form to a product method

Hello List, I have to pass an instance of a self-defined class within a form to a product method. I try it in the following way: <form action="addUploadFile" enctype="multipart/form-data" target="main" method=post> <input name="doc:instance" value="<dtml-var doc>" type="hidden"> <input name="addFile" value="add File" type="submit"> </form> But within the method addUploadFile, always the parameter doc is passed as a string (the name of the instance) Does anybody know, what I have to change, that it works? Many thanks in advance Ulla.