[c-nsp] Why it won't route vlan 1 ?

[jcovini at free.fr]

Running 12.2.18SXF8 on sup720-3b/6506

Router(config)#int gi1/1
Router(config-if)#no switchport
Router(config-if)#int gi1/1.1
Router(config-subif)#encapsulation isl 1
Command rejected: VLAN 1 cannot be allocated because it is a default VLAN
Router(config-subif)#encapsulation dot1Q 1 native
Command rejected: VLAN 1 cannot be allocated because it is a default VLAN

However this was working like a charm on older IOS versions, or onto other
platforms, so is it a platform limitation ? Is it due to 12.2.18SXF8 ? Should I
consider a change towards SXD train ? I am heritating of this design and need to
route all my .1 subinterfaces, without renumbering them if possible.

cheerios