I had the same thing happen. It turns out that number with the dashes was not the number provided by the test. When I was breaking the long arg into individual digits, I was doing (int) n%10 instead of (int) (n%10), which was not giving the desired digits (with some being negative numbers), causing the stack trace later on.

I have just tried a few solutions and it worked fine. I don't understand why you had that...

No, not my printing. Codewars is probably running a wrong test?
yes, it's Java

OK but is it your printing? The inputs are numbers, not strings. Java?

basic tests pass, seeing this with the 'Run Suite'.

Yes, it's weird. Basic tests or rando tests?

For input string: "6834172015468-4-3-92-4"

Is it your printing?

The Suite fails with a following:

For input string: "6834172015468-4-3-92-4"
test(ToSmallestTest)

Hm.... How???? :)