Tables value reverts to default if set equal to zero

When the number of tables value is set to zero, and then the node is edited, the code sets the number of tables back to the default of 18. I believe this is related to the following code in reservation_form():

  $form['tables'] = array (
    '#type' => 'textfield',
    '#title' => t('Number of tables'),
    '#default_value' => ($node->tables ? $node->tables : 18)
  );

When the value of $node->tables is zero, then it is reset to the default of 18. If the user only wants chair rows, and no tables, this is a problem. I think the default of 18 should only be set when a new node is created, not when an existing one is edited.