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.

function performExercise(count: Int, type: String) -> String {

    var output: String = ""

    for _ in 0 ..< count {

        output += "I'm doing a \(type)\n"

    if count < 10 {

        output += "That was easy!"

    } else if count < 30 {

        output += "That was challenging!"

    } else {

        output += "Phew - I need a break!"

    return output

print(performExercise(count: 50, type: "crunch"))