Cisco Firewall :: 1812 - IPV6 - ZBF Not Working

I'm having several issues with IPv6 and ZBF. I've narrowed one of them to a very simple setup. I tried 15.1(4)M in GNS2 and 15.1(4)M3 on a 1812.
 
The setup is
 
[PC] ----- [R1] ------ [R2]
 
R1 and R2 are interconnected by IPv6 only and there is a tunnel over that link to carry IPv4 from R2 to PC.

And I'm trying to ping PC from R2 with IPv4. We're looking at the R1 config mainly. (the other are included for completeness)
 
When I try to ping the PC from R2, I get this on R1 console:
 
%FW-6-DROP_PKT: Dropping icmp session [::]:0 [::]:0 on zone-pair zp_vpn_to_lan class cm_icmp   with ip ident 0
 
Which really doesn't make much sense because zone vpn and zone lan are purely IPv4 and should never see IPv6 traffic as such ...
 
If I remove all ZBF related config, then traffic flows without problem.
 
R1 config
 
class-map type inspect match-all cm_icmp
match protocol icmp
 
policy-map type inspect pm_icmp
class type inspect cm_icmp
  inspect (code)