Error using Flann matcher for BOWImgDescriptorExtractor in opencv-python

I am using opencv3 and python to implement the bag of words model for images. Here is my code.

import numpy as np import cv2 import glob img_cls = '/home/user/class_path' imgs = glob.glob(img_cls + '*jpg') sift = cv2.xfeatures2d.SIFT_create() clusterCount = 200 termcrit = cv2.TermCriteria_EPS + cv2.TermCriteria_MAX_ITER, 10, 0.001 flags = cv2.KMEANS_RANDOM_CENTERS attempts = 10 bowTrainer = cv2.BOWKMeansTrainer(clusterCount, termcrit, attempts, flags) for i in imgs: img = cv2.imread(i, 0) kp = sift.detect(img) kp, des = sift.compute(img, kp) bowTrainer.add(des) voc = bowTrainer.cluster() FLANN_INDEX_KDTREE = 0 index_params = dict(algorithm = FLANN_INDEX_KDTREE, trees = 5) search_params = dict(checks = 50) matcher = cv2.FlannBasedMatcher(index_params, search_params) bowDE = cv2.BOWImgDescriptorExtractor(sift, matcher) bowDE.setVocabulary(voc) img = cv2.imread('/home/user/test_img.jpg', 0) kp = sift.detect(img) bdes = bowDE.compute(img, kp) After running the program, I get the following error. Could anyone point out what I might be doing wrong.

OpenCV Error: Assertion failed (The data should normally be NULL!) in allocate, file /home/user/opencv/modules/python/src2/cv2.cpp, line 163
Traceback (most recent call last):
   File "ImgBow.py", line 31, in 
     bowDE.setVocabulary(voc)
cv2.error: /home/user/opencv/modules/python/src2/cv2.cpp:163: error: (-215) The data should normally be NULL! in function allocate