How can a None type variable has an attribute `shape'?

[Qinka]

nn-common-modules/nn_common_modules/modules.py

Line 412 in 0c4eb5f

:param out_block: Tensor for skip connection, shape = (N x C x H x W), defaults to None

nn-common-modules/nn_common_modules/modules.py

Line 422 in 0c4eb5f

unpool = self.unpool(input, indices, out_block.shape)

The variable out_block is an optional parameter, and the default value is None. However, the code in line 422, out_block.value ,try to access the attribute shape of out_block.