Vignetting with CPL ?

I wonder what causes the vignetting-like effect on the right side of the tower. I assume this is due to CPL I was using ? What's the best way to fix this ? (I can't see it in the viewfinder so rotating CPL doesn't do much good).

Thanks !

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Ar eyou referring the darker shade area on top right? I don't see anything wrong with the building..... I like the pic. If it is bugging you that much, CS3 it out using the healing tool. (I think thats the tool)

Sorry, I wasn't very clear; I was only talking about the sky, not the building. I have a few photos where sky changes from light to deep blue - this is probably one of the more noticeable ones - and I am curious if this is due to CPL.

A CPL doesn't affect the whole sky uniformly; you'll see transitions, especially with wider angle lenses like you used here.

OK, thanks. That's what I thought but wasn't sure. I don't mind the effect, just wanted to know what's causing it.