This was a bug in the List MathObject answer checker. It turns out that in the case where an entry in the list is constant and produces an error when computed (like the illegal power), the error was not properly reported. I have fixed the problem, and you can get the patch by updating your copy of pg/lib/Value/AnswerChecker.pm to the current HEAD version.
I've also changed the error message you complained about. It occurs in three different files, so you need to update pg/lib/Parser/BOP/power.pm, pg/lib/Value/Real.pm, and pg/macros/contextFraction.pl to get that update.
Thanks for reporting the error.