Form element with type = value odd behaviour
You can use form properties:

$form['#mystore'] = $fields;


The form properties are not sent to the client, but are available in the submit hander. For example, core use $form['#node'], $form['#user'], etc.

Updated:

http://api.drupal.org/api/drupal/developer!topics!forms_api_reference.html#val


  Note that as of Drupal 6, you can also simply store arbitrary
  variables in $form['#foo'] instead, as long as '#foo' does not
  conflict with any other internal property of the Form API.


Updated 2:

Your problem may be related to this issue: http://drupal.org/node/1381830