Rules and Token Conversion Issue

Notice: Array to string conversion in RulesTokenEvaluator->evaluate() (line 203 ...)

I have the following code in a computed_field, its saving the values correctly and showing it in the display without issue.

$participant = array_pop(array_pop(field_get_items($entity_type, $entity, 'field_activity_participants')));
$email = db_query("select field_broker_email_email from field_data_field_broker_email where entity_id = :entity_id limit 1", array(":entity_id" => $participant))->fetchField();
$entity_field[0]['value'] = $email;

In my Rule I have the token for the field set as follows:

[activity:field-email]

I've tried

[activity:field-email:0]

Neither of them works. Any advice or suggestions arr welcomed.. Is this a bug?

Thanks