ZSH and Passwordless access with SSH in a shared home environment

Question

I have a small cluster with some nodes which share their home folder through NFS. I allow passwordless SSH access to the users with this script:

if [[ -e $homeDirs/$username/.ssh/id_rsa.pub ]]; then
    echo "User '$username' already has a public key"
else
    su $username -c "ssh-keygen -N ''"  
fi

cat $homeDirs/$username/.ssh/id_rsa.pub >> $homeDirs/$username/.ssh/authorized_keys
chown $username:$username $homeDirs/$username/.ssh/authorized_keys
chmod 600 $homeDirs/$username/.ssh/authorized_keys

This has been working properly so far, but now I would like to use zsh shell instead of bash, and it doesn't work anymore. Any clues?

Thanks!!