Lowest base system

Your task is determine the lowest number base system in which the input n ( base 10 ), expressed in this number base system, is all 1s in its digits.

See some examples:

7 in base 2 is 111 - fits! answer is 2

21 in base 2 is 10101 - contains 0, does not fit
21 in base 3 is 210 - contains 0 and 2, does not fit
21 in base 4 is 111 - contains only 1 it fits! answer is 4

n is always less than Number.MAX_SAFE_INTEGER or equivalent.