Encountering "Fatal error: Index fell out of range" error when random number is generated

I'm trying to create a bingo number generator app, however after generating numbers "Fatal error: Index fell out of range" was encountered randomly on line "let letter = prefixes[bingoBall/15]".

var numbersSeen = Set<Int>()
var prefixes = ["B-", "I-", "N-", "G-", "O-"]

func randomNumber() -> Int {

    var nextNum : Int
    repeat {
        nextNum = Int.random(in: 1...75)

        if !numbersSeen.contains(nextNum) {
            numbersSeen.insert(nextNum)
            return Int(nextNum)
        }

    } while true
}

func randomBall() {
    let bingoBall = randomNumber()
    let letter = prefixes[bingoBall/15]
    lblNumber.text = "\(letter)\(bingoBall)"

    lblNumCalled.text = String(numbersSeen.count) + " numbers called."
    lblNumRemain.text = String(75 - numbersSeen.count) + " numbers remaining."

    if lblNumPrev.text != "" {
        lblNumPrev.text = lblNumPrev.text!+", "+lblNumber.text!
    }else{
        lblNumPrev.text = lblNumber.text!
    }
}