Why does my SURF.detectAndCompute returns none before the array?

Question

I am trying to extract key point and descriptors but my SURF.detectAndCompute returns None in front of the array and because of that I cannot do the clustering. Below is my code:

image_paths = []
image_group_id = []
image_group_names = os.listdir(train_path)
descriptors = []
SURF = cv2.xfeatures2d.SURF_create()
SURF.setHessianThreshold(5000)
for group_id, train_group_path in enumerate(os.listdir(train_path)):
    image_path = train_path + '/' + train_group_path
    for path in os.listdir(image_path):
        full_image_path = image_path + '/' + path
        image_paths.append(full_image_path)
        image_group_id.append(group_id)
        _, des = SURF.detectAndCompute(cv2.imread(full_image_path), None)
        descriptors.append(des)

And this is the of the descriptors that I tried to print:

[None, array([[ 0.00140431,  0.00414569,  0.00156097,  0.00508978, -0.01072729,
         0.03109567,  0.01673188,  0.04477221, -0.04659119,  0.0652261 ,
         0.04661125,  0.08301401, -0.00103816,  0.0069982 ,  0.00168295,
         0.00876924,  0.01606237, -0.00151245,  0.01966742,  0.01294267,
        -0.2951115 , -0.14716513,  0.32090443,  0.18698329,  0.21934257,
        -0.02404423,  0.29070902,  0.17538053, -0.03951943,  0.02635496,
         0.04406727,  0.02923489,  0.03092461, -0.04000381,  0.03236163,
         0.04539306, -0.26897454, -0.112547  ,  0.33476326,  0.29709056,
         0.2916087 , -0.01106649,  0.351532  ,  0.19272245, -0.07418256,
        -0.04663706,  0.07452377,  0.04671762,  0.00109221,  0.00404389,
         0.0019812 ,  0.0101865 ,  0.00381939, -0.01452297,  0.05944061,
         0.05199388,  0.0035292 , -0.04607308,  0.03296318,  0.04701661,
        -0.00757545, -0.00286426,  0.00760168,  0.00306828]],
      dtype=float32)

The None isn't supposed to be there. What can I do to fix this? I am new in this field, any advice is very appreciated. Thank you!