Nowhere in the code you've shown do you show the array place or the function place. [There is a place function in Control System Toolbox but I don't think that's what you're calling in this context.] If it's an array, look at the code that computes it to determine how an element becomes non-positive. If it's a function determine why it's returning a non-positive number.
You can set an error breakpoint to determine the specific indices or input arguments i, j, and n that result in the non-positive index.