Pafy cannot read YouTube video as 'webm'

I'm trying to use cv2 to process YouTube videos downloaded from Pafy, but whenever I try to use the format 'webm', the getbest() always returns a 'NoneType'. Any idea why or how to fix it? Thanks.

AttributeError                            Traceback (most recent call last)
<ipython-input-45-6ab69c00964e> in <module>()

      5 video = urlPafy.getbest(preftype="webm")
      6 
----> 7 test = cv2.VideoCapture(video.url)
      8 test_frames = []
      9 
 
 AttributeError: 'NoneType' object has no attribute 'url'