Fun site, but I encountered what seems to be a bug. It 500s for the word "bill" and (after some further testing) with "kill", "thrill", "will", "still", "fill"

Fun site, but I encountered what seems to be a bug. It 500s for the word "bill" and (after some further testing) with "kill", "thrill", "will", "still", "fill" and "phil". It worked with "infill" (no results), but not "refill". Any clue what is going on here?