TypeError: unsupported operand type(s) for /: 'str' and 'str'

Hi,
I am training a neural network and want to save my model state after every 100 iterations. My code snippet is:

state_dict  = model.state_dict()
for key in state_dict.keys():
			state_dict[key] = state_dict[key].to(torch.device('cpu'))
torch.save(state_dict, args.save_dir /
			'model_iter_{:d}.pth.tar'.format(i + 1))

But I am getting the following error:

 'model_iter_{:d}.pth.tar'.format(i + 1))
TypeError: unsupported operand type(s) for /: 'str' and 'str'

What can I do to resolve this? Please help.