Unable to save extra fields

[Maestro1]

Hello. I added a field year to Student model.I want to extend the app so that students can view quizzes that are just for their year of study.
In the forms.py in StudentSignUpForm I added year = forms.ModelChoiceField(
queryset=Year.objects.all(),
required=True
)
in def save(self):
student.year.add(*self.cleaned_data.get('year'))

It gives an error 'NoneType' object has no attribute 'add'
Assist in solving this or give pointers on how to solve