I've got a simple sheet metal enclosure that I'm trying to flatten.  For some reason, I keep getting an error as shown below when I unsuppress(or hit the Flatten feature).  

Thickness =2mm, Internal Bend Radius=1mm

Oddly, if I create this as a solid body and covert to sheet metal, it works just fine and flattens as shown below

Is there something I'm missing in the sheet metal part that's causing it not to flatten?

Any help would be appreciated.