filter issue

[Matthew Whittaker-Williams]

Hi 

I am having some issues with a filter,  any advice on whats wrong with this :

# BGP Exception prefixes
filter exceptions {
        asnums = [ 286 ];
        if ( bgp_path.last ~ asnums) then accept;
}

I get the following error

bird: /etc/bird.conf, line 68: You may set only variables.

Thanks

Matt