Dense Support material is added between two parralel horizontal surfaces

[unrealwill]

Discussed on reprap forum : http://forums.reprap.org/read.php?263,564530,564530#msg-564530 (go there to see pictures)

Hello,
I'm trying to slice this part (see target.png picture) with two horizontal surfaces above each other.
The part is the "MiddleJoint" of some thingiverse object http://www.thingiverse.com/thing:989937

I'd like to have some support material sandwiched in between the two surfaces.

The slicing generates a dense (100% infill) material in between the two horizontal surface which is not some support material, (I have tried to check and uncheck "don't support bridges option" but it doesn't make a difference").
It seems as if the top of the bottom surface is repeated until the bottom of the top surface.

I'm using Slic3r version 1.2.9
Can you give me the correct settings to use to get a print ?

Thank you
William

[gongloo]

FWIW, I'm having the same issue.