Setting prefix on a ModelForm causes is_valid() to return False

[Mark Tranchant]

Setting the prefix attribute on a ModelForm causes validation to fail. Here's a model and its associated form in the project I'm working on. Note that I've excluded the two foreign keys from the ModelForm, leaving just the text.

class RptTField(models.Model):
        report = models.ForeignKey(Rpt)
        config = models.ForeignKey(RptCfgTField)
        text = models.TextField()


class RptTForm(forms.ModelForm):
        class Meta:
                model = RptTField
                exclude = ('report','config')


>>> from myproj.models import *
>>> a = RptTForm({'text':'hello world'})
>>> a.prefix
>>> a.is_valid()
True
>>> b = RptTForm({'text':'hello world'},prefix='abc')
>>> b.prefix
'abc'
>>> b.is_valid()
False

I can't see anything in the docs that explains this behaviour. I need this as I'm bundling multiple ModelForms into one HTML form, and need to put the prefix in when binding to the POST data in case validation fails and the form needs re-drawing. This is with the latest SVN, 13353.