Boost.Python.ArgumentError

[Shaw-Q]

When I am using OpenPose.detectHands():

rgb = cv2.imread('/data/cropped/%d.png' %i)
rgb = cv2.flip(rgb,1)
vec = np.array(handBB + [0, 0, 0, 0], dtype=np.int32).reshape((1, 8))       
op.detectHands(rgb, vec)

It gives an error:

Boost.Python.ArgumentError: Python argument types in
OpenPose.detectHands(OpenPose, NoneType, numpy.ndarray)
did not match C++ signature:
detectHands(class OpenPoseWrapper {lvalue}, class cv::Mat, class cv::Mat)
detectHands(class OpenPoseWrapper {lvalue}, class cv::Mat)

Does anyone know the reason or how to solve this problem?
Thanks