Auto Mask Settings

I need to darken the sky of an image that have as foreground a piece of a building.
I have created a Local Adjustment Layer and masked the sky with the Auto Mask. It looks everything ok.
Then I decrease the Exposure of the layer and it actually looks ok. But If I zoom 100%, I can see that the area between the building edge and the mask it keep about two pixels of the original sky color.
See the issue here: https://dl.dropbox.com/u/18811392/automask.jpg

Any idea what I can do to solve this issue?

Thanks.

Antonio