Another BIGNUMS bug, hurray. (sarcasm) :
Why is this happening.
that's not a bignums problem, it's that the value isn't text. You have to convert it to text first using the join block
I think the split by block should convert its first input to text (could be by invoking the toString() method) instead of doing a type check that Jens says slows down Snap!
I don't think it's doing a type check; it's the Javascript SPLIT method throwing an error. The trouble with having bignums as a library rather than built into Snap! is that it's fragile; there are probably many cases of user errors (which this is) that give rise to unexpected error message texts. It's bad enough having to override the arithmetic functions without also having to override text functions!