swift playground - Sequence of elements, but "join is unavailable: call the 'joinWithSeparator()'" error

Question

Why do I get the error "join is unavailable: call the joinWithSeparator()" at line 16 (the last line below) when I try to run it on playground? And, how can I fix it?

class Person {
    var firstName: String?
    var lastName: String?
    let gender = "female"

    func fullName() -> String {
        var parts: [String] = []

        if let firstName = self.firstName {
            parts += [firstName]
        }

        if let lastName = self.lastName {
            parts += [lastName]
        }
        return " ".join(parts)
    }
} 

Answer 1

The error message tells you what the problem is, and it tells you how to fix it. Read the error message! Do what the error message says!

return parts.joinWithSeparator(" ")