Spot the error

This Swift code contains one error – can you select the line that is incorrect?

Tip: You should mentally run the code from top to bottom, meaning that if line 3 says an integer should be returned and line 10 tries to return a string, line 10 is the wrong one.

func rateStaff(value: Int) -> String {

    var output: String = "SCORE \(value)\n"

    switch value {

    case 0...20:

        output += "Terrible"

    case 21...40:

        output += "Poor"

    case 41...60:

        output += "Average"

    case 61...80:

        output += "Good"

    default:

        output += "Excellent"

    return output

print(rateStaff(value: 31)