I had the same error using an UpdateView
I had this:
if form.is_valid() and form2.is_valid():
form.save()
form2.save()
return HttpResponseRedirect(self.get_success_url())
and I solved just doing:
if form.is_valid() and form2.is_valid():
form.save()
form2.save()
return HttpResponseRedirect(reverse_lazy('adopcion:solicitud_listar'))