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 displayError(error: Int) -> String {

    var str: String = "ERROR \(error)'

    str += "\nPlease try again\n"

    return str

}

print(displayError(error: 91))