Index: wp-includes/class-wp-customize-manager.php
===================================================================
--- wp-includes/class-wp-customize-manager.php	(revision 21021)
+++ wp-includes/class-wp-customize-manager.php	(working copy)
@@ -31,6 +31,8 @@
 		require( ABSPATH . WPINC . '/class-wp-customize-section.php' );
 		require( ABSPATH . WPINC . '/class-wp-customize-control.php' );
 
+		add_filter( 'wp_die_handler', array( $this, 'wp_die_handler' ) );
+
 		add_action( 'setup_theme',  array( $this, 'setup_theme' ) );
 		add_action( 'wp_loaded',    array( $this, 'wp_loaded' ) );
 
@@ -53,6 +55,18 @@
 	}
 
 	/**
+	 * Return the AJAX wp_die() handler if it's a customized request.
+	 *
+	 * @since 3.4.0
+	 */
+	public function wp_die_handler() {
+		if ( isset( $_POST['customized'] ) )
+			return '_ajax_wp_die_handler';
+
+		return '_default_wp_die_handler';
+	}
+
+	/**
 	 * Update theme modifications for the current theme.
 	 * Note: Candidate core function.
 	 * http://core.trac.wordpress.org/ticket/20091
@@ -967,4 +981,4 @@
 		return '#' . $unhashed;
 
 	return $color;
-}
\ No newline at end of file
+}