I've taken a series of pictures of the night sky, and took 10 dark frames (same conditions and settings as light frames, cap on), 10 bias (back inside the house, cap on, manual mode, same aperture, focus and ISO, 1/8000s) and 10 flats (inside the house, same aperture, focus, ISO, Av mode with +1 EV compensation, shot an evenly-lit cream-colored smooth wall), stacking them in DSS.
The resulting image shows a distinct over-correction of vignetting. I tried then processing it just with light and dark frames, and aside from the expected vignetting, I get normal results.
Where is the over-correction coming from, and how do I fix it? 
