program error when trying to stack matrices

I'm trying to write a short function in Prime 7.0 that will loop through data and populate multiple matrices and then stack them.
Even though the matrices are the same size, I am getting a "The dimensions of these arguments do not match." error when using the stack function.

I tried pulling sample matrices out of the loop and stacking them. That worked, as expected.

Can you not use the stack function in a loop? Or am I missing something else?

Thanks!