Django required field in model form

I have a form where a couple of fields are coming out as required when I don't want them too. Here is the form from models.py

class CircuitForm(ModelForm):
    class Meta:
        model = Circuit
        exclude = ('lastPaged',)
    def __init__(self, *args, **kwargs):
        super(CircuitForm, self).__init__(*args, **kwargs)
        self.fields['begin'].widget = widgets.AdminSplitDateTime()
        self.fields['end'].widget = widgets.AdminSplitDateTime()

In the actual Circuit model, the fields are defined like this:

begin = models.DateTimeField('Start Time', null=True, blank=True)
end = models.DateTimeField('Stop Time', null=True, blank=True)

My views.py for this is here:

def addCircuitForm(request):
    if request.method == 'POST':
        form = CircuitForm(request.POST)
        if form.is_valid():
            form.save()
            return HttpResponseRedirect('/sla/all')
    form = CircuitForm()    
    return render_to_response('sla/add.html', {'form': form})

What can I do so that the two fields aren't required?