OP solved it, closing

You're overriding global i variable.

Test suite has a bug and it gets tripped up by a global variable, which your solution happens to use (and modify). Remove global variables from your code (there is one), and all should be OK.

Stability is usually a trait of a sorting algorithm, so sort method can be described as "unstable" (if it's implemented in terms of unstable sorting algorithm), but this does not apply to every.

The reason of your error is that for some reason you call every onnull, and you have to find out why. Your solution probably incorrectly handles some edge cases.