sscan commond no feedback

[thinksource]

I try to use sscan, but it already return None, my python version is 3.6

c=tornadoredis.Client(host='localhost', port=6379)
c.connect()
c.sadd("testset", "additional")
y=c.sscan("testset", 0)

when I try to print y, it is empty, where is the return?