Why combined query doesn't have :available-fields? #cascalog

gistfile1.clj

(let [source1 [[1 2 3]]
        source2 [[4 5 6]]
        fields ["?a" "?b" "?c"]
        query (combine
                (<- fields
                    (source1 :>> fields))
                (<- fields
                    (source2 :>> fields)))]
    (:available-fields query))
=> nil