diff --git a/calendar-bundle/src/Resources/contao/dca/tl_calendar_events.php b/calendar-bundle/src/Resources/contao/dca/tl_calendar_events.php
index <HASH>..<HASH> 100644
--- a/calendar-bundle/src/Resources/contao/dca/tl_calendar_events.php
+++ b/calendar-bundle/src/Resources/contao/dca/tl_calendar_events.php
@@ -311,7 +311,7 @@ $GLOBALS['TL_DCA']['tl_calendar_events'] = array
 			'inputType'               => 'imageSize',
 			'options'                 => System::getImageSizes(),
 			'reference'               => &$GLOBALS['TL_LANG']['MSC'],
-			'eval'                    => array('rgxp'=>'digit', 'nospace'=>true, 'helpwizard'=>true, 'tl_class'=>'w50'),
+			'eval'                    => array('rgxp'=>'natural', 'includeBlankOption'=>true, 'nospace'=>true, 'helpwizard'=>true, 'tl_class'=>'w50'),
 			'sql'                     => "varchar(64) NOT NULL default ''"
 		),
 		'imagemargin' => array
[Calendar] Add an empty option to the image size select menu (see #<I>)