TypeError: <type 'set'> is wrong data type

def checkio(first,second):
    a = set(first.split(","))
    b = set(second.split(","))
    return a & b

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I would like to give some feedback about ...

From: http://www.checkio.org/mission/common-words/solve/

HTTP_USER_AGENT:

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_1) AppleWebKit/601.2.7 (KHTML, like Gecko) Version/9.0.1 Safari/601.2.7