[Nut-upsuser] What does "[d] unrecognized" mean?

[Timur Tabi]

# upsdrvctl start
Network UPS Tools - UPS driver controller 2.7.4
Network UPS Tools - APC Smart protocol driver 3.1 (2.7.4)
APC command table version 3.1
[d] unrecognized

I see this error every time I start NUT, but I can't find any 
documentation about it.

I assume it's from this code in apcsmart.c:

static inline void warn_cv(unsigned char cmd, const char *tag, const 
char *name)
{
         if (tag && name)
                 upslogx(LOG_WARNING, "%s [%s] - %s invalid", name, 
prtchr((char)cmd), tag);
         else
                 upslogx(LOG_WARNING, "[%s] unrecognized", 
prtchr((char)cmd));
}

I guess "d" is some kind of command, but that's all I can glean from this.