Index: wp-includes/theme.php
===================================================================
--- wp-includes/theme.php	(revision 20994)
+++ wp-includes/theme.php	(working copy)
@@ -1129,6 +1129,9 @@
 		$attachment = " background-attachment: $attachment;";
 
 		$style .= $image . $repeat . $position . $attachment;
+	} else if ( get_theme_support( 'custom-background', 'default-image' ) ) {
+		$image = " background-image: none;";
+		$style .= $image;
 	}
 ?>
 <style type="text/css" id="custom-background-css">