Found this in failed armhf build log, where quickcheck found a way to break
prop_idempotent_key_decode. The "s" indicates size, but since nothing comes
after it, that's not valid. When encoding the resulting key, no size was
present, so it encoded to "a--a".
Also, "a-sX--a" is not legal, since X is not a number. Not found by
quickcheck.
e8ab82390e