samba - Jan 2008 - srv_spoolss_nt.c "Failed to send message about upgrading driver"

After trying to upgrade a printer driver, I get the message 
"_spoolss_addprinterdriver: Failed to send message about upgrading driver 
[ ]!"

This is the section of code shown below from srv_spoolss_nt.c that 
generates the message.
if (!srv_spoolss_drv_upgrade_printer(driver_name)) {
                DEBUG(0,("_spoolss_addprinterdriver: Failed to send 
message about upgrading driver [%s]!\n",
                        driver_name));

It seems that the value of driver_name is null.

Could anyone help me figure out the cause of this error?