opencv with raspberry step #3 houghlines

opencv step #3 houghlines

[Twiskful]

First part of programme stops and gets error at:" a, b, c = lines, shape AttributeError: "NoneType" object has no attribute "shape"". Sometimes it goes longer than usual, could you get me some help please? EDIT: I added "if lines is not None: " before the place where error starts just as in next video.