tun0 unsuitable for nmap?

Not sure where the problem is, so I just ask it here.

nmap has not been working for me since I switched to using user-ppp
with tun0. Though I am connected to the unternet, nmap would report:

leafy_at_chihiro:~$ sudo nmap -v -sS <hostname>

Starting Nmap 4.10 ( http://www.insecure.org/nmap/ ) at 2006-06-26 17:22 CST
WARNING: Unable to find appropriate interface for system route to <my
dsl ip per tun0>

nexthost: failed to determine route to <ip for remote hostname>
QUITTING!

Routing table shows:

leafy_at_chihiro:~$ netstat -r
Routing tables

Internet:
Destination        Gateway            Flags    Refs      Use  Netif Expire
default            203-70-98-1.adsl.d UGS         0   984046   tun0
localhost          localhost          UH          0   239838    lo0
192.168.0          link#2             UC          0        0   fxp1
203-70-98-1.adsl.d 203-70-98-57.adsl. UH          1        0   tun0

Is there anyway I can make this work?

Regards,

Jiawei
-- 
"Without the userland, the kernel is useless."
               --inspired by The Tao of Programming