How to prevent shape layer masks from creating a contour?

Hey there! 🙂 

I just wonder... when I mask an image by using a shapes' alpha channel it will create an ugly contour (it seems that the shape layer won't make a 100% overlay above the image below it and that the edge of the original image below comes through, without the colored and multiplied layer style of that shape mask layer above it)

Is there any way to prevent that? the actual contour of the shape is fully disabled.