error: (-215:Assertion failed) (npoints = prevPtsMat.checkVector(2, CV_32F, true)) >= 0 in function 'calc'

[Archangel31]

I am getting the following error for my video file.
error: (-215:Assertion failed) (npoints = prevPtsMat.checkVector(2, CV_32F, true)) >= 0 in function 'calc'
Here is the code:
old_gray = cv2.cvtColor(frames[0],cv2.COLOR_BGR2GRAY)
old_features = cv2.goodFeaturesToTrack(old_gray,**self.features)

    for frame_num in range(1,len(frames)):
        frame_gray = cv2.cvtColor(frames[frame_num],cv2.COLOR_BGR2GRAY)
        new_features, _,_ = cv2.calcOpticalFlowPyrLK(old_gray,frame_gray,old_features,None,**self.lk_params)

Please help!