undefined method M_Membership::current_subscription()

Hi there,

I get the following error when trying to use the following function:

Error:
Fatal error: Call to undefined method M_Membership::current_subscription() in

Function:

function get_sub_by_user($user_id) {
$member = new M_Membership( $user_id );
	if($member->has_subscription()) {
		// note this could return more than one subscription so may want to check it's an array and then return the first one if only want one result<br />
		return $member->current_subscription();
	} else {
	return false;
	}
}

I am using a multisite install where the Membership plugin is network activated.

Thanks,
Michael