watmap NullPointerException on line 244

[alysiceknife]

Hello! Apologies if this issue isn't super well-formatted, as I'm new to GitHub. I'm trying to make a Watmap image and am having issues with an unexplained NullPointerException error on line 244 after the image names are prepared, or partway through the matching process, seemingly at random. I've checked my work over and over on renaming the files correctly, and have tried different image sets. The images I'm currently using are titled as follows "iconXX.png", and the relevant section of the code is formatted like this:

final static String pattern_prefix = "./icon/icon";
final static String file_ext = ".png";
final static int pattern_init = 1; // starting number
final static int pattern_length = 18; // how many images from the set
final static int pattern_size = 2; // number of digits

I'm not sure what could be causing this error, since it happens during the matching process. I am using a fresh version of the script downloaded from GitHub today.