Index exceeds matrix dimensions when running "keyDots"

Dear Psychtoolbox Group,

I am getting the error "Index exceeds matrix dimensions" when I attempt to run the keyDots.m program from the Variable Coherence Random Dot Motion code provided at https://www.shadlenlab.columbia.edu/resources/VCRDM.html. 
What is the best way to debug the program, and would you happen to know possible indices that might exceed matrix dimensions? 
Thank you, in advance, for your help!
Sarah