Chromakey inverted


I am trying to use the advanced chromakey function, but I get a strange effect - the subject rather than the background is getting keyed out (I am using a green backdrop). I thought unclicking the ‘Invert’ button would do the trick, but nothing happens!

I have previously used this function before with OK results (on the same piece of video), but this is driving me up the wall!

Any ideas?