I know that I must be missing something simple.  The attached file consists of an extruded block, and a 3D sketch.  The sketch is unsolvable.  It is a line with both ends having an 'On Plane' sketch relation.  Both are on the same plane.  I get this a lot, and have no idea why.  Seems completely broken to me, but I always assume operator error first.  Anyone else seen this happen?  Any idea why?

Thanks,

Mark