An apartment building i lived in 5 years ago, recently sent me a bill saying i owe them $186

It it my understanding that this debt is past the statue of limitations(3 years) for that state(north carolina) and they should not be able to collect this from me. They never once contacted me between the time i moved out, to now. Out of the blue i get a bill for $186. I do not want my credit to be hurt in anyway because of this, but i also do not believe i should have to pay. They're saying the $186 is for the cost of cleaning the apartment after i moved out. What should i do?