Notice: Undefined index: #attributes in theme_container() + Recoverable fatal error: Argument 1 passed to drupal_attributes()

Error message on page where must be my settings form:

Notice: Undefined index: #attributes in theme_container() (line 3234 of /www/includes/form.inc).
Recoverable fatal error: Argument 1 passed to drupal_attributes() must be an array, null given, called in /www/includes/form.inc on line 3234 and defined in drupal_attributes() (line 2296 of /www/includes/common.inc).

Here is the code which generate this error, but why i don't know.

mymodule.module:

function mymodule_menu() {
  $items = array();

  $items['admin/config/system/mysettings'] = array(
	'title' => 'My module settings',
	'description' => 'This is my module settings.',
	'page callback' => 'mymodule_admin',
	'file' => 'mymodule.admin.inc',
	'type' => MENU_NORMAL_ITEM,
	'access arguments' => array('access administration pages'),
  );

  return $items;
}

mymodule.admin.inc:

function mymodule_admin() {
	$form = array();

	$form['mymodule_settings'] = array(
		'#type' => 'fieldset',
		'#title' => t('my module settings'),
	);

	return system_settings_form($form);
}

Is it normal? Or my mistake?
And nevermind what type of form element you will use. Error is coming again and again.