Edge line showing using transparency mask

[Telury]

I'm trying to make a transparency mask to hide part of an object (Can't use pathfinder for this one), but everytime I try to make the mask it shows a border line even if the mask is larger than my object. I can see it if I do a huge zoom and also when I export the image.

What I'm doing wrong?

[Ton Frederiks]

What are you masking with what?

Does the line also export when you choose Art Optimized Anti-aliasing?