uqi returning nan

[rengel8]

I'm working with two images, transformed to tensors in the shape of (1x3x500x500). I'm following the code examples for each metric from its web-page, but I do get nan for uqi, also for different "image"-pairs. Is there something else I might have missed? Or is there a pool of test files, that work like expected?

A few other metrics work with the same tensors; preds, target.
ergas 50.0
lpips 0.1482967
ms_ssim 0.97375298
psnr 29.01943779
ssim 0.8981939

uqi nan

uqi = UniversalImageQualityIndex()
uqi = uqi(preds, target)

Any advice or help is much appreciated. Regards, Rainer