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 transformValue(width: Int) -> String {

    var returnValue: Int = "The width is "

    returnValue += String(width)

    return returnValue

}

print(transformValue(width: 21))