giving initial value raises 'value for field is required' error in django

giving initial value raises 'value for field is required' error in django

Django form trips me many times...
I gave initial value to a ChoiceField
self.fields['thread_type'] = forms.ChoiceField(choices=choices,
widget=forms.Select,
initial=thread_type)
The form which is created with thread_type with the above code doesn't
pass is_valid() because 'thread_type' isn't provided.
(The error message is not in english so I can't give the exact error in
english)
What should I do to pass the is_valid() call?