I haven’t had a chance to take my new Atik16ic out for a test run yet, but I did try a dark frame and I’m a little concerned. Is it normal to see a darkframe like this?

This was 20 seconds with the lens cover on. I was hoping for a completely black image and was very surprised to find this light area across the image. I ran several test frames using  ArtemisCapture and this image was one of the last when the camera had been on for several minutes. The image was taken indoors, the room temperature was around 20°C any feedback would be very helpful.