Need Help with vision.BlockMatcher

I tried to run the blockmatcher example in https://www.mathworks.com/help/vision/ref/vision.blockmatcher-system-object.html. At the line motion = step(hbm,img1,img2); I got the error Error using vision.BlockMatcher/validateInputsImpl Expected input I to be a non-empty matrix.

I don't know where does this I come from. I need to use the blockmatcher to on my video analysis project, but I don't know what happen.

Thanks.

[Navan Ruthramoorthy]

The error means, image input one (img1) is empty. This object expects that input to be a non-empty matrix.