python3 AttributeError: 'NoneType' object has no attribute 'split'

 1 from wsgiref.simple_server import make_server
 2  
 3 def RunServer(environ, start_response):
 4     start_response('200 ok',[('Content-Type','text/html')])
 5     return '<h1>Hello world</h1>'
 6 
 7 if __name__ == '__main__':
 8     httpd = make_server('127.0.0.1',9000,RunServer)
 9     print('servering HTTP on port 9000')
10     httpd.serve_forever()

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