init queryset return 'expected string or buffer'

Hi
my queryset return this error

expected string or buffer

class Reserve(forms.ModelForm):
    food_name = forms.ModelChoiceField(queryset=Food.objects.all())

    def __init__(self, year=None, month=None, day=None, serve_date=None, *args, **kwargs):
        super(Reserve, self).__init__(*args, **kwargs)
        self.year = year
        self.month = month
        self.day = day
        self.serve_date = serve_date
        date_stamp =  time.strptime(serve_date,"%Y-%m-%d")
        serve_date = datetime.date(*date_stamp[:3])
        self.fields['food_name'].queryset = Food.objects.filter(
            serve_date__year = year, serve_date__month = month, serve_date__day = day)

    class Meta:
        model = Reservation
        fields = ('food_count', 'food_name')

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