not an issue.

It is pretty much efficient brute force.
I've worked on this kata for hours, trying to find a mathematical relationship between the numbers.
There were none, that is, none that were particularly noteworthy.

Problems involving sums of digits usually don't have much mathematical reasoning behind them.

Just try to find a way to limit the number of numbers you test.

IT's difficult to tell wothout seeing your code, or even without knowing the language you use. But my guess is that you use substring somewhere and pass in invalid arguments into it. It's jsut a guess tho.