Prefix X:X:X:X::1/64 overlaps with another prefix

[mediterr]

Hi

I have one problem when i configure ipv6 address on the interface fast 0/1 of the cisco 2621XM ios Version 12.3(7)T.

i have one tunnel with prefix /48

interface Tunnel1
no ip address
ipv6 address X:X:X::2/48
tunnel source x.x.x.x
tunnel destination x.x.x.x
tunnel mode ipv6ip

And when i configure ipv6 address with prefix /64 on the interface fast 0/1 the router display: "Prefix X:X:X:X::1/64 overlaps with another prefix" the address of interface 0/1 is for my subnet

I have another router cisco 2651 with ios 12.2(2)T and this permit to configure address /64 and /48

why cant i do it? How can i do it???