Django passing kwargs to inherited form causes error (generic UpdateView)

When I use django generic view - UpdateView and try to update form kwargs like this:

#views.py

from .forms import SomeInheritedForm
from .models import SomeModel

from django.views.generic import UpdateView

class SomeUpdateView(UpdateView):
    model = SomeModel
    form_class = SomeInheritedForm

    def get_form_kwargs(self, **kwargs):
        kwargs = super(SomeUpdateView, self).get_form_kwargs(**kwargs)
        kwargs['workspace'] = 'whatever'
        return kwargs

#forms.py

from .models import SomeModel

from django import forms

class ParentForm(forms.Form):
    def __init__(self, *args, **kwargs):
        self.workspace = kwargs.pop('workspace', None)
        super(ParentForm, self).__init__(*args, **kwargs)

class InheritedForm(forms.ModelForm, ParentForm):
    class Meta:
        model = SomeModel

This will give such an error:

Django Version: 1.7.2
Exception Type: TypeError
Exception Value:    
__init__() got an unexpected keyword argument 'workspace'
Exception Location: /[ommited]/my_venv/local/lib/python2.7/site-packages/django/views/generic/edit.py in get_form, line 45
Python Executable:  /[ommited]/my_venv/bin/python
Python Version: 2.7.3

However, if I declare __init__ in InheritedForm it will work fine. But then, with more inherited forms I would have to copy the code, which is against DRY principle.