Insecure destination failure

Question

I have one struct, and sometimes for some users it will crash when trying to access a variable of this type.

struct AppSettings {
    var mute:Bool {
        didSet {
           if mute != oldValue {
               let savedSettings = NSUserDefaults.standardUserDefaults()
               savedSettings.setBool(mute, forKey: KEY_SETTING_MUTE)                   
           }
        }
    }

    init() {
        let savedSettings = NSUserDefaults.standardUserDefaults()
        if let savedMute = savedSettings.objectForKey(KEY_SETTING_MUTE) as? Bool {
            mute = savedMute
        } else {
            mute = false
        }      
    }

}
var appSettings = AppSettings()

And somewhere during application launch it sometimes crashes

if appSettings.mute {  // This will sometimes cause a crash
}                      // in AppDelegate or the methods it calls

This is for some users only and I cannot play it. Not being a reproduction is the worst, because it leaves me nothing wrong with the job.

Searching unsafe mutable addressoras an error does not help, because there are almost no results.