BokehDOF problem in the dark

When I use BokehDOF it is not possible to have perfectly dark image.
In this image you see the problem (note : The luminosity of the image is slightly retouched to make it obvious)
If I turn of DOF the image is dark.
I guess it comes from the Bokeh Shape.

Is there is workaround to have perfect dark with BokehDOF?