Trim end off of string in swift, getting error

Am making a calculator app in swift, once my answer is obtained I want to display it in a UILabel. Only problem is I want to limit said answer to 8 characters. Here is my code:

let answerString = "\(answer)"
    println(answer)
    calculatorDisplay.text = answerString.substringToIndex(advance(answerString.startIndex, 8))

This does not return any compiler errors but at runtime I get:

fatal error: can not increment endIndex

Any and all help would be greatly appreciated.