Problem Getting mutual friends of logged in facebook account and other account FBML

Question

Hey,

Well at the current moment I'm attempting to get the mutual friends between two users, and then pick a random one of those. It then gets the name of the random mutual friend, but it's not working for some reason.

here's what I have (where $lg_userid is the logged in users id, and $target_id is the friend, I then get the mutual friends between them and pick a random one)

$common = $facebook->api_client->friends_getMutualFriends($lg_userid,$target_id);
$one_common = array_slice(array_shuffle($common), 0, 1);
$common_user_details = $facebook->api_client->users_getInfo($one_common, array('first_name'));
$firstnamecommon = $common_user_details['first_name'];

It's giving me this error

Error message: The underlying FQL query made by this API call has encountered the following error: Can't lookup all friends of (not logged in user id); can only lookup for the logged in user (logged in user) or for pairs of users Error code:13

Not really sure what I'm doing wrong, could anyone help me out?

Thanks!