BokehDOF problem in the dark

When I use BokehDOF it is not possible to have perfectly dark image.
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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?

Hugo.