tcpdump on loopback device

[Graham Menhennitt]

I've successfully installed tcpdump from the 3rd party software
repository and it works fine for monitoring the en0 interface. However,
when I try to monitor the loopback device, it gives an error and exits.
This is on QNX 6.3.0 using tcpdump 3.7.1 with libpcap 0.7.

# tcpdump
tcpdump: listening on en0
^C
0 packets...
# tcpdump -i en0
tcpdump: listening on en0
^C
0 packets...
# tcpdump -i lo0
tcpdump: BIOCSETIF: lo0: No such device or address
# /sbin/ifconfig
lo0: flags=...
en0: flags=...
#

Does anybody have any ideas?

Thanks,
Graham