J.C. Penney (JCP) is adopting a one-year "poison pill" to ward off hostile takeovers after investor William Ackman took a large stake in the company.

Ackman and his hedge fund Pershing Square Management on Oct. 8 disclosed a nearly 17% stake in J.C. Penney. Pershing Square owns about 39.1 million shares of Penney stock, according to a filing with the Securities and Exchange Commission. The same day, Vornado Realty Trust disclosed a 9.9% stake and said it would consult with Pershing Square.

The department store chain said Monday that it enacted the shareholder rights agreement because of recent stock purchases and to ensure fair treatment of stockholders, a clear reference to Ackman's move.