TypeError whic shouldn't happen

I would like to give some feedback about ...

From: http://www.checkio.org/mission/place-queens/solve/

In case when my function should return empty set i get error type - despite that I convert returned variable into set! I tried: K=[] K=set(K) return K

K=[] K=set(K) return set(K)

K=[] return set(K)

K=set() return K

return set()

But nothing works!

HTTP_USER_AGENT:

Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Dragon/46.9.15.424 Chrome/46.0.2490.86 Safari/537.36