trying to use batch_update()

I am trying to update a field in all of the record showing with this:

$this->db->where('email', $_SESSION['userid']);
$this->db->where('campaign', $_SESSION['campaign']);
$data[] = array(
    'in_sample' => 0
);
$this->db->update_batch('employees', $data);

The where()'s are used to make sure that I only update this user's data since this is a multi-user system. When I run the update_batch() line I get this error. In sample is an int btw. 

A Database Error Occurred
You must specify an index to match on for batch updates.

Filename: models/MyModel.php
Line Number: 143



I have set in_sample to be indexed in the database.

proof that an old dog can learn new tricks