Converting to EXT4 on encrypted lvm

[Andrig T. Miller]

After upgrading to Fedora 11 x86_64, I started to look into converting
to EXT4 from EXT3.

My / partition is an lvm that is encrypted (I do have a separate /boot).

I tried following the instructions using tune2fs -O
extents,uninit_bg,dir_index /dev/sda2 (my / device and /boot is
/dev/sda1).

I get the following error:

tune2fs 1.41.4 (27-Jan-2009)
tune2fs: Bad magic number in super-block while trying to open /dev/sda2
Couldn't find valid filesystem superblock.

Finally, I just booted by changing /etc/fstab to specify ext4, which
works fine, but from reading the various instructions, I won't have
the new features turned on unless I am able to use tune2fs
successfully.

Is the error I'm getting because its an encrypted volume? Is there
different instructions for an encrypted volume?

Thanks for any help.