--- Operating System, Graphics card ---

Windows 7 64-bit, nvidia gtx 570

--- Blender version with error, and version that worked ---

both official release and r59008 have issue

--- Short description of error ---

I will preface this by saying I am unfortunately unable to narrow down any further what is causing the issue than this:

I have a Vertex Weight Mix modifier under a subsurf modifier. I am mixing vertex group 'Left' which has all weights at 1 with vertex group 'Right' which has all weights at 0. There is also a texture mask present which is blank. The current file (attached) will crash or lock up as soon as the 'Right' vertex group is chosen from the drop down list.

If either the subsurf modifier or texture mask is not present the crash will not occur.
If both vertex groups have been chosen and there is a subsurf modifier *below* the Vertex Weight Mix modifier then moving the subsurf *above* the modifier will crash.
If both vertex groups have been chosen and the current subsurf is removed and a new one added and moved *above* then it will crash after switching from 'Catmull-clark' to 'Simple' subdivision type and back again.
If both vertex groups have been chosen and the current texture is removed then re-choosing the texture from the down down box will cause blender to crash.
If the 'Right' vertex group's weights are set to anything but fully zero, the crash will not occur.


--- Steps for others to reproduce the error (preferably based on attached .blend file) ---

1. Choose the vertex group called 'Right' in the 'Default Weight B' dropdown box on the 'Vertex Weight Mix' modifer using the attached file.