2.4.19-pre8: nf_hook: hook 0 already set

I've seen this twice in /var/log/messages, every time just after establishing a PPP link:

May 27 23:48:14 espoo pppd[11626]: pppd 2.4.1 started by ppp, uid 15
May 27 23:48:14 espoo pppd[11626]: Using interface ppp0
May 27 23:48:14 espoo pppd[11626]: Connect: ppp0 <--> /dev/pts/0
May 27 23:48:14 espoo pppd[11626]: CHAP peer authentication succeeded for iapetus.localdomain
May 27 23:48:15 espoo pppd[11626]: found interface eth0 for proxy arp
May 27 23:48:15 espoo pppd[11626]: local  IP address 172.17.1.64
May 27 23:48:15 espoo pppd[11626]: remote IP address 172.17.3.1
May 27 23:48:15 espoo pppd[11626]: Deflate (15) compression enabled
May 27 23:51:18 espoo kernel: nf_hook: hook 0 already set. 
May 27 23:51:18 espoo kernel: skb: pf=0 (unowned) dev=eth0 len=46 

Frank