Deleted unwanted selections/slices in the sprite editor?

When I slice up my sprite sheet, it gives me unwanted (blank) selections/slices. When I select them and hit delete (on the mac) nothing happens (just get the didn’t work sound effect). How can I delete them?

Hold the command button then hit delete.

i am using OSX sierra and the way to delete it is pressing command+backspace. hope works (works for me)