他動詞や自動詞が補語に名詞を取るときその名詞は無冠詞になるという規則があります。 結構、複数形にしてしまう間違いが多いポイントです。頭に入れておきましょう。
turn+whistle-blower(無冠詞名詞)→A series of product mislabeling scandals has probably made the managers of many food makers very much worried about the possibility that some of their employees turn whisle-blower.
take+hostage(無冠詞名詞)→The Japanese Prime Minister reiterated that he had no intention of wiyhdrawing the SDF contingent (部隊) from Iraq when a Japanese civilian was taken hostage and his life threatened by terrorist.
The guerrillas took (held) the embassy staff members hostage (as hostages).