Drawing partially transparent gradient

I am trying to draw an NSGradient starting from a light gray (though  
that's irrelevant) to clearColor over a (light) background. However,  
for some reason the gradient becomes dark in the middle, while near  
the ends it looks as I expected. I already tried setting the  
compositeOperation explicitly to NSCompositeSourceOver, but that did  
not help (other operations are certainly wrong). Also playing with the  
CG blend mode got me nowhere.

Would anyone have any idea what I could have done wrong?

thanks,
Christiaan