c++ - boost::assign::map_list_of 列挙型の値

Question

boost::assign::map_list_of が an_enum is not of class type エラーを生成するのはなぜですか? 例えば

exp.aggregates = (
                       boost::assign::map_list_of
                       ("a",static_cast<an_enum>(3)),               
                       ("b",static_cast<an_enum>(4)),
                       ("c",static_cast<an_enum>(0))
                 ).operator ltr_and_enum()