diff --git src/wp-includes/class-wp-customize-manager.php src/wp-includes/class-wp-customize-manager.php
index 0052a1a..2cdf311 100644
--- src/wp-includes/class-wp-customize-manager.php
+++ src/wp-includes/class-wp-customize-manager.php
@@ -424,7 +424,7 @@ final class WP_Customize_Manager {
 	 * @since 3.4.0
 	 */
 	public function after_setup_theme() {
-		$doing_ajax_or_is_customized = ( $this->doing_ajax() || isset( $_SERVER['customized'] ) );
+		$doing_ajax_or_is_customized = ( $this->doing_ajax() || isset( $_POST['customized'] ) );
 		if ( ! $doing_ajax_or_is_customized && ! validate_current_theme() ) {
 			wp_redirect( 'themes.php?broken=true' );
 			exit;