Can't create an object with a foreign key

I get the error when I try to make an object.

Cannot assign "u'Category object'": "Course.category" must be a "Category" instance.

here is my admin form

admin.py

admin.site.register(Category, list_display = ('name',))      CATEGORIES = [(category, category.name) for category in Category.objects.all()]    class CourseAdminForm(forms.ModelForm):      class Meta:          model = Course      category = forms.ChoiceField(choices=CATEGORIES)    class CourseAdmin(admin.ModelAdmin):      form = CourseAdminForm   

admin.site.register(Course, CourseAdmin) 

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