Type problem with psnr

I'm using the command psnr on a reference image and a changed image, but I'm getting a class error. When I run the code, I get the error message that input type "double" isn't an input argument. So I tried im2uint8, but that's also an undefined argument. I have the image processing toolbox, so I'm not sure what the problem is.

[Geoff Hayes]

Brian - in the Command Window type

 which -all psnr

to see which directory the psnr function can be found in. What do you see when you run this command?