iCloud UIDocumentPickerViewController is empty on real iPhone 5

I want to use the UIDocumentPickerViewController to select a ".json" file. In the simulator this works fine but not on a real device :(

Image: http://i.imgur.com/bR0cRXn.jpg

@IBAction func btnImport(sender: AnyObject) {
    let documentPicker = UIDocumentPickerViewController(documentTypes: [kUTTypeJSON as NSString], inMode: .Import)
    documentPicker.delegate = self
    presentViewController(documentPicker, animated: true, completion: nil)
}

func documentPicker(controller: UIDocumentPickerViewController,
    didPickDocumentAtURL url: NSURL) {
        // Do something
        println(url)
}