Error using Block Matcher

I'm brand new to the computer vision toolbox, and I'm attempting to use the vision.BlockMatcher command to analyze a piece of each frame of a video. I've been sticking pretty close to the formatting used by mathworks in their examples, but I'm receiving an error message that I cannot explain. it informs me:

Error using vision.BlockMatcher/step Too many input arguments; expected 1 (in addition to the object handle), got 2.

my code:

while ~isDone(videoFileReader);

      videoFrame = step(videoFileReader);

      Zone = imcrop(videoFrame,[X4(1),X4(2),X4(3),X4(4)])

      motion = step(hbm,Background,Zone);

end

The thing that confounds me is that the mathworks example looks exactly the same to me:

3.Compute motion of the two images.

motion = step(hbm, img1, img2);

Anyone have an idea of what I might be doing wrong??

[Anand]

Can you please provide more context? What's in variables hbm, Background, Zone? What are you trying to achieve? Which example are you looking at?

[Walter Roberson]

Also which MATLAB version are you using?