diff --git a/src/wp-includes/theme.php b/src/wp-includes/theme.php
index 85119ed..0f3b5f4 100644
--- a/src/wp-includes/theme.php
+++ b/src/wp-includes/theme.php
@@ -1902,12 +1902,20 @@ function _wp_customize_include() {
 		|| ( is_admin() && 'customize.php' == basename( $_SERVER['PHP_SELF'] ) )
 	) )
 		return;
+	wp_bootstrap_customize_manager();
+}
+add_action( 'plugins_loaded', '_wp_customize_include' );
 
+/**
+ * Include the WP_Customize_Manager class and instantiate it into the wp_customize global.
+ *
+ * @since 4.0.0
+ */
+function wp_bootstrap_customize_manager() {
 	require( ABSPATH . WPINC . '/class-wp-customize-manager.php' );
 	// Init Customize class
 	$GLOBALS['wp_customize'] = new WP_Customize_Manager;
 }
-add_action( 'plugins_loaded', '_wp_customize_include' );
 
 /**
  * Adds settings for the customize-loader script.