[Nut-upsuser] Premature end of script headers: upsimage.cgi

[benjamin thielsen]

hi-

i'm using version 2.4.1, courtesy of debian testing, and noticed that  
in the web interface, the image for the battery voltage bar graph  
wasn't displaying.  the other images (change, input, output, load)  
seem to be fine.  the browser reports "500 internal server error", and  
apache reports "Premature end of script headers: upsimage.cgi,  
referer: https://graphs.ltn.lvc.com/cgi-bin/nut/upsstats.cgi?host=ups6@localhost 
"

upsc seems to also be working as expected, and reports a value for  
battery voltage:

 >upsc ups6 at localhost battery.voltage
55.00

i have 5 of these upses in total (smart-ups 1500 xlm monitored via  
snmp) which all exhibit the same behavior.

how can i further troubleshoot this?

thanks
-ben