[PyMOL] PyMOL movie question

[Jennifer Ekstrom]

Hello, 
I'm a new PyMOL user, and am trying to make my first molecular movie.  

I've inserted the following into my script: 

        mset 1 x120                  
        util.mroll(1,120,1)          
        set ray_trace_frames=1
        set cache_frames=0
        mclear
        mpng mov 

and I'm getting the following output:

PyMOL>mset 1 x120                  
PyMOL>util.mroll(1,120,1)          
PyMOL>set ray_trace_frames=1
 Setting: ray_trace_frames set to 1.00000.
PyMOL>set cache_frames=0
 Setting: cache_frames set to 0.00000.
PyMOL>mclear
PyMOL>mpng mov 
Traceback (most recent call last):
  File "C:\Program Files\DeLano Scientific\PyMOL/modules\pymol\parser.py",
lin
e 126, in parse
    result=apply(kw[nest][0],args[nest],kw_args[nest])
  File "C:\Program Files\DeLano Scientific\PyMOL/modules\pymol\moving.py",
lin
e 178, in mpng
    r = cmd._mpng(prefix,int(first)-1,int(last)-1)
ValueError: invalid literal for int(): mov0002.png


Any suggestions? 

Thanks, 
Jennifer