In cognitive radio networks with primary users implementing CSMA/CA, such as 802.11 technologies, when all the primary users are in backoff states, the secondary users may detect primary users as inactive and try to access the channel, which will prolong primary users backoff procedure. This phenomenon is called hidden collision. In this paper, we first demonstrate how primary users' performance is degraded by hidden collision. Then, to protect primary users from excessive hidden collision, we propose a method to prevent secondary users from accessing the time slots during which all active primary users are in backoff states. Numerical examples are provided to show the effectiveness of the proposed solution.