Index: wp-admin/custom-header.php
===================================================================
--- wp-admin/custom-header.php	(revision 23185)
+++ wp-admin/custom-header.php	(working copy)
@@ -1009,6 +1009,7 @@
 		}
 
 		set_theme_mod( 'header_image', $default );
-		set_theme_mod( 'header_image_data', (object) $default_data );
+		if ( ! empty( $default_data ) )
+			set_theme_mod( 'header_image_data', (object) $default_data );
 	}
 }