Drupal 7 Apachesolr: hook_apachesolr_query_prepare() not being called?

Question

I've been struggling with this for a few hours now. I've tried everything I can think of but can't seem to get the hook hook_apachesolr_query_prepare() to fire. Here is the code I'm calling in my template.php file:

/**
* Implements hook_apachesolr_query_prepare().
*/
function pz_apachesolr_query_prepare(DrupalSolrQueryInterface $query) {
    echo "Query Prepare Test";
    $query->setAvailableSort('entity_id', array(
        'title' => t('Node ID'),
        'default' => 'asc',
    ));
    $query->setAvailableSort('ds_changed', array(
        'title' => t('Changed Date'),
        'default' => 'asc',
    ));
 }

hook_apachesolr_query_alter() works fine, but I don't think I can add sort options (which is my ultimate goal here).

I've tried:

• Completely clearing and reindexing the site
• Flushing all caches multiple times
• Commenting out cache in the solrconfig.xml files
• Disabling and re-enabling search modules

I'm on Drupal 7 using Apachesolr 7.x-1.7.

I'm sure it's something simple I'm missing