How to set form field attributes on a django inline form

Question

So i'm trying to disable some form field widgets in an inline form (by setting disabled attribute in rendered html).

The following is a minified version of some code that seems to half work:

class IncomingItemForm(forms.ModelForm):
    class Meta:
        model = models.IncomingItem
    def __init__(self, *args, **kwargs):
        super(IncomingItemForm, self).__init__(*args, **kwargs)
        if self.instance.id is not None:
            self.fields[some_field_name].widget.attrs.update({'disabled' : 'disabled'})

class IncomingItemInline(admin.TabularInline):
    model = models.IncomingItem
    form = IncomingItemForm

What happens when i do this is: some_field_name is disabled for all but the first inline row.

I did some debugging by trying to disable only one row and it seems that this is an off-by-one error. Is this a django.contrib.admin bug or am i doing something stupid?

Edit: I'm using django 1.7b4