Hi, I believe I have found, somewhat of a work around to this bug.
If you get said error, all you need to do is manipulate the roatation/position of the bone and select the mesh and hit "auto" button.
I find the easiest way to do this is as follows:
1)create an IK between for the bones that the mesh should be weighted to (hopefully there is only two)
2)jump into to animation (this way you don't need to loose your set up pose)
3)position the IK so there is a bend at the joint
4) hit "auto" button with the mesh selected
It seems, atleast for me, to be caused by not selecting the weight tool (duh!). Seems logical right? however you don't have to be in the weight tool to manually edit the verts weights... which may be a bug itself