diff --git a/wp-includes/theme.php b/wp-includes/theme.php
index 87a3a2bf2..3f55ece98 100644
--- a/wp-includes/theme.php
+++ b/wp-includes/theme.php
@@ -1231,7 +1231,7 @@ function preview_theme_ob_filter_callback( $matches ) {
 	)
 		return $matches[1] . "#$matches[2] onclick=$matches[2]return false;" . $matches[4];
 
-	$link = add_query_arg( array('preview' => 1, 'template' => $_GET['template'], 'stylesheet' => @$_GET['stylesheet'] ), $matches[3] );
+	$link = add_query_arg( array( 'preview' => 1, 'template' => $_GET['template'], 'stylesheet' => @$_GET['stylesheet'], 'preview_iframe' => 1 ), $matches[3] );
 	if ( 0 === strpos($link, 'preview=1') )
 		$link = "?$link";
 	return $matches[1] . esc_attr( $link ) . $matches[4];