diff --git a/wp-includes/class-wp-customize-manager.php b/wp-includes/class-wp-customize-manager.php
index 60303e6714..c14d61652a 100644
--- a/wp-includes/class-wp-customize-manager.php
+++ b/wp-includes/class-wp-customize-manager.php
@@ -63,9 +63,9 @@ final class WP_Customize_Manager {
 	protected $registered_control_types = array();
 
 	/**
-	 * $_POST values for Customize Settings.
+	 * Unsanitized values for Customize Settings parsed from $_POST['customized'].
 	 *
-	 * @var array
+	 * @var array|false
 	 */
 	private $_post_values;
 
@@ -75,11 +75,11 @@ final class WP_Customize_Manager {
 	 * @since 3.4.0
 	 */
 	public function __construct() {
-		require( ABSPATH . WPINC . '/class-wp-customize-setting.php' );
-		require( ABSPATH . WPINC . '/class-wp-customize-panel.php' );
-		require( ABSPATH . WPINC . '/class-wp-customize-section.php' );
-		require( ABSPATH . WPINC . '/class-wp-customize-control.php' );
-		require( ABSPATH . WPINC . '/class-wp-customize-widgets.php' );
+		require_once( ABSPATH . WPINC . '/class-wp-customize-setting.php' );
+		require_once( ABSPATH . WPINC . '/class-wp-customize-panel.php' );
+		require_once( ABSPATH . WPINC . '/class-wp-customize-section.php' );
+		require_once( ABSPATH . WPINC . '/class-wp-customize-control.php' );
+		require_once( ABSPATH . WPINC . '/class-wp-customize-widgets.php' );
 
 		$this->widgets = new WP_Customize_Widgets( $this );
 
@@ -399,23 +399,46 @@ final class WP_Customize_Manager {
 	}
 
 	/**
-	 * Decode the $_POST['customized'] values for a specific Customize Setting.
+	 * Parse the incoming $_POST['customized'] JSON data and store the unsanitized
+	 * settings for subsequent post_value() lookups.
+	 *
+	 * @since 4.1.1
+	 *
+	 * @return array
+	 */
+	public function unsanitized_post_values() {
+		if ( ! isset( $this->_post_values ) ) {
+			if ( isset( $_POST['customized'] ) ) {
+				$this->_post_values = json_decode( wp_unslash( $_POST['customized'] ), true );
+			}
+			if ( empty( $this->_post_values ) ) { // if not isset or of JSON error
+				$this->_post_values = false;
+			}
+		}
+		if ( empty( $this->_post_values ) ) {
+			return array();
+		} else {
+			return $this->_post_values;
+		}
+	}
+
+	/**
+	 * Return the sanitized value for a given setting from the request's POST data.
 	 *
 	 * @since 3.4.0
+	 * @since 4.1.1 Introduced 'default' parameter.
 	 *
 	 * @param WP_Customize_Setting $setting A WP_Customize_Setting derived object
-	 * @return string $post_value Sanitized value
+	 * @param mixed $default value returned $setting has no post value (added in 4.2.0).
+	 * @return string|mixed $post_value Sanitized value or the $default provided
 	 */
-	public function post_value( $setting ) {
-		if ( ! isset( $this->_post_values ) ) {
-			if ( isset( $_POST['customized'] ) )
-				$this->_post_values = json_decode( wp_unslash( $_POST['customized'] ), true );
-			else
-				$this->_post_values = false;
+	public function post_value( $setting, $default = null ) {
+		$post_values = $this->unsanitized_post_values();
+		if ( array_key_exists( $setting->id, $post_values ) ) {
+			return $setting->sanitize( $post_values[ $setting->id ] );
+		} else {
+			return $default;
 		}
-
-		if ( isset( $this->_post_values[ $setting->id ] ) )
-			return $setting->sanitize( $this->_post_values[ $setting->id ] );
 	}
 
 	/**
diff --git a/wp-includes/class-wp-customize-setting.php b/wp-includes/class-wp-customize-setting.php
index 0207b709fd..d8bf1e2371 100644
--- a/wp-includes/class-wp-customize-setting.php
+++ b/wp-includes/class-wp-customize-setting.php
@@ -103,12 +103,18 @@ class WP_Customize_Setting {
 		return $this;
 	}
 
+	protected $_original_value;
+
 	/**
 	 * Handle previewing the setting.
 	 *
 	 * @since 3.4.0
 	 */
 	public function preview() {
+		if ( ! isset( $this->_original_value ) ) {
+			$this->_original_value = $this->value();
+		}
+
 		switch( $this->type ) {
 			case 'theme_mod' :
 				add_filter( 'theme_mod_' . $this->id_data[ 'base' ], array( $this, '_preview_filter' ) );
@@ -159,7 +165,15 @@ class WP_Customize_Setting {
 	 * @return mixed New or old value.
 	 */
 	public function _preview_filter( $original ) {
-		return $this->multidimensional_replace( $original, $this->id_data[ 'keys' ], $this->post_value() );
+		$undefined = new stdClass(); // symbol hack
+		$post_value = $this->manager->post_value( $this, $undefined );
+		if ( $undefined === $post_value ) {
+			$value = $this->_original_value;
+		} else {
+			$value = $post_value;
+		}
+
+		return $this->multidimensional_replace( $original, $this->id_data['keys'], $value );
 	}
 
 	/**
@@ -425,8 +439,15 @@ class WP_Customize_Setting {
 			$node = &$node[ $key ];
 		}
 
-		if ( $create && ! isset( $node[ $last ] ) )
-			$node[ $last ] = array();
+		if ( $create ) {
+			if ( ! is_array( $node ) ) {
+				// account for an array overriding a string or object value
+				$node = array();
+			}
+			if ( ! isset( $node[ $last ] ) ) {
+				$node[ $last ] = array();
+			}
+		}
 
 		if ( ! isset( $node[ $last ] ) )
 			return;