[PyMOL] cannot 'fit' a particular pdb file

[Michael Bovee]

Hi,
I have loaded 3 pdb files of homologous forms of an enzyme. The 'fit' 
command works for superimposing/aligning two of them, but the third 
file always gives a No Atoms Selected error. Is there something in the 
pdb file I can fix? As a general rule now, I Hide Everything at first 
and then Show Lines as I begin with a new PyMOL session. I can see all 
three molecules, just can't align the third one (EcHRSade) with the 
first two (TtHisRS and SaHRS).

PyMOL>fit TtHisRS, SaHRS
 Executive: RMS =    4.222 (718 to 718 atoms)

PyMOL>fit TtHisRS, EcHRSade
ExecutiveRMS-Error: No atoms selected.
PyMOL>fit EcHRSade, TtHisRS
ExecutiveRMS-Error: No atoms selected.
PyMOL>fit EcHRSade, SaHRS
ExecutiveRMS-Error: No atoms selected.

Thanks,
--Michael