NAT-TRAVERSAL on the FWSM - regular translation creation failed for protocol 50

Hi All,

I am trying to permit a inside client to establish a VPN through my FWSM to a VPN server on the outside. However the connection keeps failing due to error: "%FWSM-3-305006: regular translation creation failed for protocol 50 src...".

I cannot enable NAT-TRAVERSAL as it is telling me it is not a supported feature. I assume this means either the feature does not exist on the FWSM or it is automated in some way.
fwsm(config)# isakmp nat-traversal
This feature is not supported on this platform.
fwsm(config)#

i have permitted isakmp, 500, 4500 on udp.

How might I fix this problem.

Many thanks