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 processValue(depth: Int) -> String {

    var result: String = "The depth is "

    result += String(depth

    return result

}

print(processValue(depth: 31))