i found a fix for this problem here: https://gist.github.com/ethitter/4430520
i just used an or, so it should be backward compatible, replace gist-fix line 19 with this:
if ( (isset( $_POST['customize'] ) && isset( $_POST['theme'] ) && $_POST['customize'] == 'on') || ( is_a( $wp_customize, 'WP_Customize_Manager' ) ) )