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 checkSpeed(value: Int) -> String {

    var str: String = "Result for \(value) - "

    if value < 10 {

        str += "that seems low"

    } else if value < 20 {

        str += "that's about right"

    } else {

        str += "that seems high"

    return str

print(checkSpeed(value: "10"))