如何获得Facebook电子邮件价值

Question

let profile = Facebook(clientId: "xxx", clientSecret: "xxx")
let fullName = try? profile.fullName()
let gender = try? profile.gender()
let avatar = try? profile.pictureURL()
let email = try? profile.email()

在最后一行崩溃。我没有设置我的Facebook电子邮件值，这也许是崩溃的原因。获取电子邮件值的正确方法是什么？