i fixed it

Nope, the solution is valid. The description of the problem states that the input will contain strings and non-negative integers. There's no reason to check for non-negativeness because we already know that we will receive non-negative integers.

Oh I see, what you're saying. I'll update the test to make sure the input data is changed.

Can you elaborate what you mean by "while even without change of input value (matrix value), the output is marked as correct"? I don't know how I would check that the user only modified the existing matrix instead of creating a new one. The way I wrote this kata assumes a bit of an honor code.