It's debatable whether Rapids is supposed to be immune to stack traces from strings it doesn't currently like. I'm reminded of early parsing days when we said "we don't care about random datasets"...until we cared.
In any case, I think this function is legal and it gets an assertion error in Frame.java
test_rapids_funs_basic2.py
the other passes.
No matter what, I think h2o should give "unsupported" exception or silently wrong answers for all unsupported strings. (not NPE or assertions), with a bias to the former when easy.