And what does your solution return in your IDE for incrementString("foobar000")?

If your code fails on Codewars but seems to work in your IDE, it usually does not mean that the code is correct. It means that you are testing it in the IDE incorrectly.

count the zeroes in both strings.

• please do not use the issue tag to ask for help, ask a question instead
• i ran the sample tests against your code and saw this:

Incorrect answer for input: "foobar000"
Expected: equal to          "foobar001"
Actual:                     "foobar0001"