Shelling problem

 From:  Dan (CORNYSH)
3424.1 
Hi, I'm trying to shell the attached shape. It seems to a proper solid yet it doesn't seem to shell properly (modus operandi: shell, then make a hole in a face by doing a boolean diff with another solid to see inside). I have tried with various thicknesses but it doesn't work. Is it because the shape is too complex?

Thanks
Dan
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