Schema alter add callback for none existed fields

Hello.

This code makes troubles because it add export callback for none existed fields.
we have that troubles with simpletest clone module, because it use schema to create db copy sql requests.

/**
 * Implementation of hook_schema_alter().
 */
function input_formats_schema_alter(&$schema) {
....
  $schema['filter_formats']['fields']['format']['no export'] = TRUE;
  $schema['filter_formats']['fields']['settings']['export callback'] = 'input_formats_export_settings';
  $schema['filter_formats']['fields']['filters']['export callback'] = 'input_formats_export_filters';

we have not this fields in filter_formats schema

 $schema['filter_formats'] = array(
    'description' => 'Stores input formats: custom groupings of filters, such as Filtered HTML.',
    'fields' => array(
      'format' => array(
        'type' => 'serial',
        'not null' => TRUE,
        'description' => 'Primary Key: Unique ID for format.',
      ),
      'name' => array(
        'type' => 'varchar',
        'length' => 255,
        'not null' => TRUE,
        'default' => '',
        'description' => 'Name of the input format (Filtered HTML).',
      ),
      'roles' => array(
        'type' => 'varchar',
        'length' => 255,
        'not null' => TRUE,
        'default' => '',
        'description' => 'A comma-separated string of roles; references {role}.rid.', // This is bad since you can't use joins, nor index.
      ),
      'cache' => array(
        'type' => 'int',
        'not null' => TRUE,
        'default' => 0,
        'size' => 'tiny',
        'description' => 'Flag to indicate whether format is cachable. (1 = cachable, 0 = not cachable)',
      ),
    ),
    'primary key' => array('format'),
    'unique keys' => array('name' => array('name')),
  );