A tension device for a corded-loop drive system has a base with a sprocket on the base such that the cord loop travels over the sprocket. At least one resilient finger has a first end attached to the base and a free end. Each finger is sized and positioned to engage the cord loop when the tension device is not properly mounted to a wall or window frame and prevent movement of the cord loop around the sprocket. There is a hole in the finger through which a screw passes. When the base is placed on a mounting surface and the screw is driven into the mounting surface, the free end of the finger will have moved from the first position to a second position away from the cord so that the cord loop may move around the sprocket and the window covering can be fully raised and fully lowered.