Problem extracting descriptors of FAST opencv3

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Hi everybody, I'm started to use openCV(3.0.0-dev) and now I'm trying different way to extract features from images (Fast, GoodFeaturesToTrack, Mser, Star, Sift, Surf). I started with SIFT and SURF and my code works good but now i'm tryng with FAST and i can't understand how extract descriptors of keypoints!

For SIFT, for example, I used this code

    detector = cv2.xfeatures2d.SIFT_create()
    kp, des = detector.detectAndCompute(img ,None)

In des i have my descriptors and it's ok, but when I've tried with FAST

detector = cv2.FastFeatureDetector_create()
kp, des = detector.detectAndCompute(img ,None)

I've obtained following error -> "OpenCV Error: The function/feature is not implemented () in detectAndCompute"

I've noted that there is the method dectect() and I can extract keypoints but what about descriptors? Why it works for SIFT/SURF and not for FAST? How can I obtain the descriptors? Thank you for answer.