Your image/layer is not formatted with square pixel aspect ratio (PAR). Your workflow to correct this is to create a comp that is the square PAR equivalent of your non-square PAR image/layer. Then fit the image/layer to match the size (CTRL+F) of this square PAR comp. Then nest this comp and apply CC Sphere.
This is another reason why folks should always work in square PAR comps. Should you require to output to a non-square PAR, then nest your square PAR into a non-square PAR comp at the last step - just prior to rendering.