using &*_expire with interval values defined before

hi everybody

does anyone know why this doesn't work and results in an error?

policy-script:
> const tcp_scan_period = 2 min &redef;
> global tcp_scan: table[addr] of count &write_expire=tcp_scan_period;

error:
> line xx (tcp_scan_period): bad tag in ExprVal::Val

thanx for any help
christoph