AttributeError: 'NoneType' object has no attribute 'span' - Pytube Module Error

I want to download youtube video, so I used pytube Library to extract video streams from youtube video URL. I installed pytube library.

pip install pytube

 I have written following script.

import pytube

yt = pytube.YouTube("https://www.youtube.com/watch?v=ijChai_OHaA")
video_streams = yt.streams.filter(res="720p")
video_streams.first().download('./videos')

 I got this following error while running the python program 

"AttributeError: 'NoneType' object has no attribute 'span'"

There is no problem with the code. I restarted my PC and rerun the program. It works fine.

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