Hey guys, I've got a quick enlarging question, I've only recently started doing it, so bit unsure!

The enlarger I use can physically take upto a 120 film (i.e the sliders move out far enough to accommodate the negative), and when the light is shone through the neg, the neg appears fine on the board/paper EXCEPT what I can only describe as 'light vignetting'. Not enough light is getting to the 4 corners, so when it comes to developing, those bits are white still(exactly same 'look' as vignetting, but white!).

Is this just a limitation of the enlarger, or am I missing something? Even if I make the enlargement as small as it will go, it still has this effect, but not as bad. 35mm is fine, it's just 120 square images that are affected. Does it need a different lens for 120?

This is the enlarger, excuse the rubbish cam phone pic (can't remember the brand of enlarger, it's the one on the left):



Any ideas?