There is another (although somewhat trivial) answer - zero. Considering the problem as a Diophantine equation, the possibility of the number having 1, 2, 3 or more digits must be considered. It can be shown fairly readily that the 1-digit solution is 0, the only 2-digit solution is 54, and there are no solutions with 3 or more digits. These are the relevant equations:
1-digit (the number is a): a = 6a
2-digit (the number is ab): 10a + b = 6(a+b)
3-digit (the number is abc): 100a + 10b + c = 6(a+b+c)
https://en.wikipedia.org/wiki/Diophantine_equation