How to insert a NULL value when using zen_db_perform() ?

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I'm currently trying to write my own product type for ZenCart 1.5. One of the fields on the collect_info page is supposed to enter a BLANK (null) value into the table. However, on the loading of the page, I noticed that instead of entering a null value, the script is inserting a value of zero (0). Does anyone know how I should I be doing the insert to make sure the NULL VALUE is inserted?

Here is the insert statement as in my update_product.php:

      //// INSERT PRODUCT-TYPE-SPECIFIC *INSERTS* HERE //////
      $cfv_data_array = array('products_id' => $products_id,
                              'grade' => zen_db_prepare_input($_POST['grade']),
                              'nation_id' => zen_db_prepare_input($_POST['nation_id']),                              
                              'clan_id' => zen_db_prepare_input($_POST['clan_id']),
                              'race' => zen_db_prepare_input($_POST['race']),
                              'rarity_id' => zen_db_prepare_input($_POST['rarity']),
                              'trigger_id' => zen_db_prepare_input($_POST['trigger_id']),
                              'skill_id' => zen_db_prepare_input($_POST['skill_id']),
                              'power' => zen_db_prepare_input($_POST['power']),
                              'shield' => zen_db_prepare_input($_POST['shield']),
                              'critical' => zen_db_prepare_input($_POST['critical']),
                              'flavor_text' => zen_db_prepare_input($_POST['flavor_text'])  
                            );            
zen_db_perform(TABLE_CFV_CARDS, $cfv_data_array);
      ////    *END OF PRODUCT-TYPE-SPECIFIC INSERTS* ////////