'NoneType' object has no attribute 'repeat'

Hi, guys, when I run my ssim function, it shows an error:‘NoneType’ object has no attribute ‘repeat’ and I don’t know what’s wrong with my code.
Two Inputs size are both 3 3 256 256

def isinstance(x, A_tuple): # real signature unknown; restored from __doc__
    Return whether an object is an instance of a class or of a subclass thereof.
    A tuple, as in ``isinstance(x, (A, B, ...))``, may be given as the target to
    check against. This is equivalent to ``isinstance(x, A) or isinstance(x, B)
    or ...`` etc.
def ssim(x: torch.Tensor, y: torch.Tensor, kernel_size: int = 11, kernel_sigma: float = 1.5,
         data_range: Union[int, float] = 1., reduction: str = 'mean', full: bool = False,
         k1: float = 0.01, k2: float = 0.03) -> Union[torch.Tensor, Tuple[torch.Tensor, torch.Tensor]]:
    if isinstance(x, torch.ByteTensor) or isinstance(y, torch.ByteTensor):
        x = x.type(torch.float32)
        y = y.type(torch.float32)

    kernel = gaussian_filter(kernel_size, kernel_sigma).repeat(x.size(1), 1, 1, 1).to(y)

It seems gaussian_filter is not returning anything, which would raise this error.
Could you check, if that might be the case?