ValueError: need at least one array to concatenate

[Hridaydeep]

def image_descriptors(file): img = cv2.imread(file, 0) img = cv2.resize(img, (256, 256)) _ , descriptors = cv2.SIFT().detectAndCompute(img, None) return descriptors

def folder_descriptors(folder): files = glob.glob(folder + "/*.jpg") print("Calculating descriptos. Number of images is", len(files)) return np.concatenate([image_descriptors(file) for file in files])

this error is showing at last line i.e. return np.concatenate([image_descriptors(file) for file in files])