gre tunnel doesn't create

[s m]

hello all

i have freebsd 9.2 which is customized. now when i want to create gre
tunnel and run "ifconfig gre0 create" command, this error is shown:

ifconfig: SIOCIFCREATE2: Invalid argument

i check this command on a fresh freebsd 9.2 and it works so my changes
cause this. i have many changes on my freebsd (install many services,
optimize kernel and ...) and i need all of them. do you have any suggestion
what change causes this error? or how can i fix it in order to be able to
create a gre tunnel?

thanks in advance,
Sam