Hmm makes sense. The issue arises when virtual_step() is called:
… in 
optimizer.virtual_step()
… line 282, in virtual_step
self.privacy_engine.virtual_step()
… line 435, in virtual_step
self.clipper.clip_and_accumulate()
…  line 179, in clip_and_accumulate
named_params=self._named_grad_samples(),
… line 263, in _named_grad_samples
where the error is thrown