Assertion error at step 13

[mickvanhulst]

Dear authors,

When running step 13.2 I get the following assertion error. Any idea why this may be the case? The code on that specific line is: assert(new_grd_pos > 0)

I would comment the code, but the variable new_grd_pos is used to write to the new file, so I am not sure that is a viable option.