django: flatpages error "Cannot use None as a query value" (En)

If you see "Cannot use None as a query value" error message in Django admin tying to save new Flatpage object, then just fill all mandatory fields. Probable you forgot to specify 'site' field.
It's very strange not to see standart django error message but the stack trace.. And it's a bug in Flatpages field validation code.
Moreover, this bug has been fixed (Ticket #18324).
p.s. stable version (c)