0.5 lens obstrction

Need help figuring out that the issue is with a 0.5 lens

Pictures end up with a black arc across the top - almost like someone took a sharpie marker and drew an arc line. Doesn't happen when using other lens. Can't see it when tak pic. Can't see anything from looking at the camera lens. The first picture with the cookie shows it although the picture is on its side. the 2nd pic shows the outside of the lens. Thank you! I just bought this on ebay from an indivdual and need to figure it out quickly.