fill command bug

I tried to use the /fill command in the nether replacing air and for a large area the game did not recognise it as air. In the picture you can see what I typed. It targetted everything around the vine, but did not actually fill it, the only bit that did get filled was a bit I had tested on by putting in blocks myself and replace those with air.
It did not work for other blocks either, as if the air was not recognised as such by the command.