Then make sure the value is valid before calling the method.

For this kata, calling count is very expensive and unnecessary. You should avoid it and either call it only as many times as required, or (even better) do not use it at all (it's not needed here).
You need to take the advantage of the fact that except the one unique number, all other numbers are equal. Using this fact, you can avoid counting them every time.

return an integer -> int(...)

Different data type.

me too :(

Thank you Hobovsky!

Please read the last bullet point in this paragraph of FAQ: https://github.com/codewars/codewars.com/wiki/Troubleshooting-your-solution#when-i-print-my-answer-it-looks-exactly-the-same-as-the-expected-output-yet-tests-fail