How do i get rid of noisy edges with a chroma key (green screen) in sony vegas or

while filming? I am trying to use a chroma key (green screen) in sony vegas and im getting a lot of noise(black spots) around the edges of the green screen. Why is this happening and how do i fix it? Im just trying to test out the effect so if i did something wrong in the actual filming(lighting wise or whatnot) then let me know so i don't do it in the future.
 
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