Index: wp-includes/class-wp-customize-manager.php
--- wp-includes/class-wp-customize-manager.php
+++ wp-includes/class-wp-customize-manager.php
@@ -34,8 +34,9 @@
 
 		add_filter( 'wp_die_handler', array( $this, 'wp_die_handler' ) );
 
+		add_action( 'init',         array( $this, 'init'        ) );
 		add_action( 'setup_theme',  array( $this, 'setup_theme' ) );
-		add_action( 'wp_loaded',    array( $this, 'wp_loaded' ) );
+		add_action( 'wp_loaded',    array( $this, 'wp_loaded'   ) );
 
 		// Run wp_redirect_status late to make sure we override the status last.
 		add_action( 'wp_redirect_status', array( $this, 'wp_redirect_status' ), 1000 );
@@ -111,33 +112,27 @@
 
 		if ( is_admin() && ! $this->doing_ajax() )
 		    auth_redirect();
-		elseif ( $this->doing_ajax() && ! is_user_logged_in() )
-		    $this->wp_die( 0 );
 
 		show_admin_bar( false );
 
-		if ( ! current_user_can( 'edit_theme_options' ) )
-			$this->wp_die( -1 );
-
 		$this->original_stylesheet = get_stylesheet();
+		$this->theme               = wp_get_theme( isset( $_REQUEST['theme'] ) ? $_REQUEST['theme'] : null );
 
-		$this->theme = wp_get_theme( isset( $_REQUEST['theme'] ) ? $_REQUEST['theme'] : null );
-
+		// Once the theme is loaded, we'll validate it.
 		if ( $this->is_theme_active() ) {
-			// Once the theme is loaded, we'll validate it.
 			add_action( 'after_setup_theme', array( $this, 'after_setup_theme' ) );
 		} else {
-			if ( ! current_user_can( 'switch_themes' ) )
-				$this->wp_die( -1 );
 
-			// If the theme isn't active, you can't preview it if it is not allowed or has errors.
+			// Bail if theme has errors
 			if ( $this->theme()->errors() )
 				$this->wp_die( -1 );
 
-			if ( ! $this->theme()->is_allowed() )
+			// Bail if theme is not allowed
+			if ( !$this->theme()->is_allowed() )
 				$this->wp_die( -1 );
 		}
 
+
 		$this->start_previewing_theme();
 	}
 
@@ -154,6 +149,34 @@
 	}
 
 	/**
+	 * Callback to perform capability checks on the current user, once
+	 * wp_get_current_user() has been called.
+	 *
+	 * @since 3.6
+	 */
+	function init() {
+
+		// Bail if user is not logged in
+		if ( $this->doing_ajax() && ! is_user_logged_in() ) {
+		    $this->wp_die( 0 );
+		}
+
+		// Bail if user cannot edit theme options
+		if ( ! current_user_can( 'edit_theme_options' ) ) {
+			$this->wp_die( -1 );
+		}
+
+		// Previewing a new theme
+		if ( ! $this->is_theme_active() ) {
+
+			// Bail if user cannot switch themes
+			if ( ! current_user_can( 'switch_themes' ) ) {
+				$this->wp_die( -1 );
+			}
+		}
+	}
+
+	/**
 	 * Start previewing the selected theme.
 	 *
 	 * Adds filters to change the current theme.