[Python] 'The parameter `image` must be a 2-dimensional array'

I'm trying to extract orb features via scikit-image, but I got the error The parameter image must be a 2-dimensional array. I converted it to greyscale so the image is actually 2-dimensional.

from skimage.feature import ORB
from skimage.color import rgb2gray

def find_orb(img, n_keypoints=2000, **kwargs):
descriptor_extractor = ORB(n_keypoints, **kwargs)
descriptor_extractor.detect_and_extract(rgb2gray(img))
return descriptor_extractor.keypoints, descriptor_extractor.descriptors

pano_image_collection = io.ImageCollection('jpeg/lowres/8_*.jpg',
load_func=lambda f:io.imread(f).astype(np.float32) / 255)
img = pano_image_collection[0]
keypoints, descriptors = find_orb(img)


And this is the error

ValueError Traceback (most recent call last)
<ipython-input-5-5dce31f8d3f4> in <module>()
----> 7 keypoints, descriptors = find_orb(img)

<ipython-input-4-26e09ccf38ce> in find_orb(img, n_keypoints, **kwargs)
14 descriptor_extractor = ORB(n_keypoints, **kwargs)
---> 15 descriptor_extractor.detect_and_extract(rgb2gray(img))
16 return descriptor_extractor.keypoints, descriptor_extractor.descriptors

/usr/local/lib/python3.6/site-packages/skimage/feature/orb.py in detect_and_extract(self, image)
302
303 keypoints, orientations, responses = \
--> 304 self._detect_octave(octave_image)
305
306 if len(keypoints) == 0:

/usr/local/lib/python3.6/site-packages/skimage/feature/orb.py in _detect_octave(self, octave_image)
139 # Extract keypoints for current octave
140 fast_response = corner_fast(octave_image, self.fast_n,
--> 141 self.fast_threshold)
142 keypoints = corner_peaks(fast_response, min_distance=1)
143

/usr/local/lib/python3.6/site-packages/skimage/feature/corner.py in corner_fast(image, n, threshold)
745
746 """
--> 747 image = _prepare_grayscale_input_2D(image)
748
749 image = np.ascontiguousarray(image)

/usr/local/lib/python3.6/site-packages/skimage/feature/util.py in _prepare_grayscale_input_2D(image)
140 def _prepare_grayscale_input_2D(image):
141 image = np.squeeze(image)
--> 142 assert_nD(image, 2)
143 return img_as_float(image)
144

/usr/local/lib/python3.6/site-packages/skimage/_shared/utils.py in assert_nD(array, ndim, arg_name)
176 raise ValueError(msg_empty_array % (arg_name))
177 if not array.ndim in ndim:
--> 178 raise ValueError(msg_incorrect_dim % (arg_name, '-or-'.join([str(n) for n in ndim])))
179
180

ValueError: The parameter `image` must be a 2-dimensional array

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