Hi (I'm the kata author), I had the same issue when I tried to submit my reference solution in 'Train' mode, however there was no error when I submitted the reference solution in kata creation mode. I ended up setting the package org.codewars.kata.dynamicjava for all of the kata classes (test class, solution, and user test class) as well as all of the test cases which seems to have fixed the problem. Try setting that package for your solution and see if you still get that error.