Check for the following line in your sim_check_options():
if (mem_lat[0] < 1 || mem_lat[1] < 1)
fatal("all memory access latencies must be greater than zero");
The line above requires that both latencies be set to a value > 0. For this assignment we want to allow mem_lat[1] to be 0, so relax the second half and just check that mem_lat[1]<0 (negative).