[PyMOL] surfaces for molecules with non-standard VDW's

In the past I have used PyMol pretty successfully to visualize low- 
resolution x-ray solution scattering results in which each "atom" in  
a pdb file is actually a large sphere (say 4-6 Angstroms)  
representing some density.  I would use

 >alter <molecule name>, vdw = 6.0

to set the sphere sizes appropriately, then compute a surface using  
the "surface" option in the "Show" pulldown of the appropriate object.

Strangely, this seems to have quit working for me. When I select  
surface, nothing happens, not even an error message.
It still works fine for older files I have, so it must be something  
particular about the geometry of the object I have (it is a rather  
sparse extended object). Anyone seen this kind of problem?

File is included below: