[Zope] assignment to form records

I've a script who gets two parameters from a form: Code (int), Fields (record). In the script, I can assign to Code, say: Code = 1 but if I try, say: Fields.a_field = 1 I get the error:

Error Type: TypeError Error Value: attribute-less object (assign or del)

Do I need some special syntax for that or is it simply not possible (and why)? Regards, Fernando