A tissue ablation probe is provided. The tissue ablation probe comprises an elongated shaft, at least one electrode tine carried by the elongated shaft, at least one tine exit from which the electrode tine(s) can be deployed from the elongated shaft and retracted within the elongated shaft, and a sheath covering the electrode tine exit(s). The sheath may, e.g., line an exterior surface of the elongated shaft or an interior surface of the elongated shaft. The sheath has at least one tine valve (e.g., a slit) positioned over the electrode tine exit(s) and configured to open when the electrode tine is deployed and to close when the electrode tine(s) is retracted. In one embodiment, the tine valve(s) is configured to open in response to pressure exerted during deployment of the electrode tine(s). In another embodiment, the tine valve(s) is configured to hinder the entry of biological material within the elongated shaft. In one embodiment, the sheath is pliable, such that the tine valve(s) can more easily hinder the entry of the biological material.