Index: wp-includes/theme.php
===================================================================
--- wp-includes/theme.php	(revision 21765)
+++ wp-includes/theme.php	(working copy)
@@ -1150,7 +1150,7 @@
 		$attachment = " background-attachment: $attachment;";
 
 		$style .= $image . $repeat . $position . $attachment;
-	}
+	} else $style .= " background-image: none;";
 ?>
 <style type="text/css" id="custom-background-css">
 body.custom-background { <?php echo trim( $style ); ?> }