Duplicity restore reports “Invalid SSH password” when I’m using a private key for connection

I'm testing the restoration of a backup with duplicity 0.6.15.

I can login to my backup server just fine using ssh and sftp using the following commands and my private key:

…sftp

root@client:~# sftp -oPort=7843 backupUser@192.168.x.x
Enter passphrase for key '/root/.ssh/id_rsa':
Connected to 192.168.x.x
sftp> exit

…ssh

root@client:~# ssh -p7843 backupUser@192.168.x.x
Enter passphrase for key '/root/.ssh/id_rsa':
Connected to 192.168.x.x
Last Login: ....

I have the following restore script used to restore the contents of the signed and encrypted backup:

#!/bin/bash
export SIGN_PASSPHRASE='<signed-key-passphrase'
export PASSPHRASE='<encryption-passphrase>'

duplicity -v9 --encrypt-key="<encryption-key-id>" --sign-key="<signed-key-id>" --force \
scp://backupUser@192.168.x.x:7843//home/backupUser/backup /mnt/restore

However when I run the script duplicity:

root@client~#: ./restore_script.sh

Duplicity gives me the following error when duplicity uses it to attempt to login to my backup server:

Using archive dir: /root/.cache/duplicity/b1a470f45b67cd7784bc8e6449383df7
Using backup name: b1a470f45b67cd7784bc8e6449383df7
Import of duplicity.backends.hsibackend Succeeded
Import of duplicity.backends.ftpbackend Succeeded
Import of duplicity.backends.botobackend Succeeded
Import of duplicity.backends.rsyncbackend Succeeded
Import of duplicity.backends.imapbackend Succeeded
Import of duplicity.backends.localbackend Succeeded
Import of duplicity.backends.giobackend Succeeded
Import of duplicity.backends.ftpsbackend Succeeded
Import of duplicity.backends.cloudfilesbackend Succeeded
Import of duplicity.backends.gdocsbackend Succeeded
Import of duplicity.backends.tahoebackend Succeeded
Import of duplicity.backends.u1backend Succeeded
Import of duplicity.backends.webdavbackend Succeeded
Import of duplicity.backends.sshbackend Succeeded
Main action: restore
================================================================================
duplicity 0.6.15 (August 19, 2011)
Args: /usr/bin/duplicity -v9 --encrypt-key=<encryption-key-id> --sign-key=<signature-key-id> --force scp://backupUser@192.168.x.x:7843//home/backupUser/backup /mnt/restore
Linux client 3.0.0-12-generic #20-Ubuntu SMP Fri Oct 7 14:50:42 UTC 2011 i686 i686
/usr/bin/python 2.7.2+ (default, Oct  4 2011, 20:03:08) 
[GCC 4.6.1]
================================================================================
Using temporary directory /tmp/duplicity-sLukkP-tempdir
Registering (mkstemp) temporary file /tmp/duplicity-sLukkP-tempdir/mkstemp-12JD0o-1
Temp has 4995375104 available, backup will use approx 34078720.
Running 'sftp  -oPort=7843 -oServerAliveInterval=15 -oServerAliveCountMax=2 backupUser@192.168.x.x' (attempt #1)
State = sftp, Before = 'Enter'
State = sftp, Before = ''
Invalid SSH password
Running 'sftp  -oPort=7843 -oServerAliveInterval=15 -oServerAliveCountMax=2 backupUser@192.168.x.x' failed (attempt #1)
Removing still remembered temporary file /tmp/duplicity-sLukkP-tempdir/mkstemp-12JD0o-1
INT intercepted...exiting.

I have a public key in my .ssh directory, and it should allow duplicity to login using that rather than asking for an ssh-password (which I have turned off on the server anyway), so I don't understand why the server is not allowing duplicity to login.

Here's my client's listing of the .ssh directory, permissions and all:

root@client:~# ls -la /root/.ssh
total 16
drwx------ 2 root root 4096 2011-11-29 01:05 .
drwx------ 8 root root 4096 2011-11-29 12:30 ..
-rw------- 1 root root 1766 2011-11-29 01:06 id_rsa
-rw-r--r-- 1 root root  442 2011-11-29 00:38 known_hosts

Is there something I'm missing? I got the backup to work just fine, and I'd like to test out restoring the backup before sealing the deal here and saying that it works.