No saddr, daddr, dport for udp.recvmsg, udp.sendmsg

[AZX AMN]

I'm running systemtap version "2.8/0.163, Debian version 2.8-2ubuntu1"
on Ubuntu 15.10, installed from the standard ubuntu packages and I'm
trying some of the examples for probing udp.recvmsg and udp.sendmsg.
I'm running this:

probe udp.recvmsg, udp.sendmsg  {
  printf(" %15s %15s  %5d  %5d  \n",
         saddr, daddr, sport, dport)
}

and most of the time only sport has a correct value while dport is 0
and saddr and daddr are 0.0.0.0. Occasionally they will have correct
values though, so it's not completely consistent.  Sometimes only
saddr and sport have correct values and the destiation is 0's. But
almost all of the time only sport is there. When I probe TCP it all
appears correctly. Are there any known reasons for this behavior for
UDP?

Thanks