According to the source code in ps_select.m from line 262~267:
if i_max > n_i
The value of i_min or i_max can not be negative if the if statement is not true. However, once the if is true, i_min will equal -n_win+1+n_i, where n_win is the parameter clap_win in step 2, which equals 32 by default, while the value of n_i depends on both the parameter filter_gird_size, which is also the parameter in step 2 and default value is 50 meters, and the size of pixels, which default is 5m20m, so, the minimum of you subset should let n_i(or n_j) equals 31, means it should larger than 3150/5=310 in col and 31*50/20=77.5 in row.
But actually I do not meet the problem as you do, so you should test it by yourself, increase the size of your subset, and maybe the problem can be solved.
Hope this helps,