time dependent ionization and cooling in a radiatively cooling gas

The most recent paper is Gnat & Sternberg, 2007, ApJS, 168, 213 on astro-ph  here

I helped Orly in this paper, and they used cloudy extensively. I have two input scripts that redo their calculation. It now fails in iteration 3 with an assert at line 231 of ion_solver.cpp

The input script for the constant density case is

title constant density time variable cooling cloud
cosmic rays background
c following two trace problem with co convergence when 
c co is turned on 
* trace ionization convergence 0 150 
* iterate 153
iterate 250
* no co molecules 
hden 0
blackbody 5,000,000K
coronal 5,000,000K init time 
ionization parameter -10 time
set nend 1
stop zone 1
print ages
time 11 to 14.0 
time 10 scale 0    
time 11 scale=0  recombination
time 20 scale=0  
end of times
c speed ups
element lithium off
element beryllium off
element boron off
element fluorine off
element phosphorus off
element chlorine off
element potassium off
element scandium off
element titanium off
element vanadium off
element chromium off
element manganese off
element cobalt off
element copper off
element zinc off
print line faint 2 log 
punch time dependent "cd.tim" 
punch cooling "cd.col"