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 takeTrain(distance: Int, destination: String) -> String {

    var str; String = "I'm going to \(destination)\n"

    str += "Distance is \(distance)km"

    return str

print(takeTrain(distance: 5, destination: "London"))