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 sectionForPage(_ pageNumber: Int) -> String {

    var result: String = "Turning to page \(pageNumber)..."

    if pageNumber < 10 {

        result += "The title is Introduction"

    } else if pageNumber < 20 {

        result += "The title is Reviews"

    } else if pageNumber < 30 {

        result += "The title is Features"

    } else {

        result += "The title is Tutorials"

    return result

print(sectionForPage(pageNumber: 16))