Setting initial value of modelForm instance

Why can't I set instance = None in my unbound model form?

def sample(request):

    prf_form = Profil开发者_如何学运维eForm(instance=None)
    print prf_form.instance #debugger print
    return render_to_response(template, locals(), context_instance=RequestContext(request))

I get the following exception:DoesNotExist

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An alternative is to instantiate an empty Profile and pass that:

profile = Profile()
prf_form = ProfileForm(instance=profile) 

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Just don't send it an instance:

def sample(request):

    prf_form = ProfileForm()
    return render_to_response(template, locals(), context_instance=RequestContext(request))