diff -r 7778a2bbdf9d -r 90493551b1eb web/formfields.py
--- a/web/formfields.py	Fri Mar 26 13:23:25 2010 +0100
+++ b/web/formfields.py	Fri Mar 26 13:33:32 2010 +0100
@@ -381,7 +381,10 @@
         for field in self.actual_fields(form):
             if field is self:
                 try:
-                    yield field, field.process_form_value(form)
+                    value = field.process_form_value(form)
+                    if value is None and field.required:
+                        raise ProcessFormError(form._cw._("required field"))
+                    yield field, value
                 except UnmodifiedField:
                     continue
             else: