[R] TreeBUGS - subscript out of bounds

[Max Hennig]

Dear all,
 
I've only (very) recently started to use R (so please be easy on me if I may omit to mention relevant information or have overlooked fairly basic steps to solving the problem, since I do not have a lot of experience) because I'm interested in multinomial processing tree modeling with the TreeBUGS package (Heck, Arnold & Arnold 2017 - TreeBUGS: An R package for hierarchical multinomial-processing-tree modeling).
I have attempted to conduct a permutation test with my dataset, as described on pages 5-6 of the paper. I've brought the data in the necessary long format (participant case in column 1, stimulus index in column 2, observed response in column 3) and specified the tree structure as described in the paper, in my case four trees with two possible responses each.
When specifying the test, I am given the message:
 
Error in M[, tree] : subscript out of bounds
 
Though I've found some information on this general error online, all of it applies to different tests and didn't help me to solve the problem.
Has anyone of you encountered this error before, or has a suggestion for me?
 
Best,
Max