[c-nsp] IPv6 address set bug?

[Hank Nussbacher]

When installing a 10G interface, we tried copying the ipv6 address from 
the GigaE port:
ipv6 address 2001:xxx:0:B::1/64
IOS 12.2(18)SXE6b says:
2001:xxx:0:B::1/64 can not be configured on TenGigabitEthernet10/1, in use
The new 10Gig port is in shutdown.  Makes no difference if we try copying 
the ipv6 address to a 10G or GigaE port.  Same result.  This does work 
with IPv4 and IOS just gives a warning like:
xxx.139.185.0 overlaps with GigabitEthernet9/20
but does do the ip address set since the interface is in shutdown.

Is this a feature of ipv6 or a bug?

Thanks,
Hank