diff --git a/wp-admin/includes/nav-menu.php b/wp-admin/includes/nav-menu.php
index be40d8565..b29a2fd0b 100644
--- a/wp-admin/includes/nav-menu.php
+++ b/wp-admin/includes/nav-menu.php
@@ -132,7 +132,7 @@ class Walker_Nav_Menu_Edit extends Walker_Nav_Menu  {
 				<p class="field-css-classes description description-thin">
 					<label for="edit-menu-item-classes-<?php echo $item_id; ?>">
 						<?php _e( 'CSS Classes (optional)' ); ?><br />
-						<input type="text" id="edit-menu-item-classes-<?php echo $item_id; ?>" class="widefat code edit-menu-item-classes" name="menu-item-classes[<?php echo $item_id; ?>]" value="<?php echo esc_attr( $item->classes ); ?>" />
+						<input type="text" id="edit-menu-item-classes-<?php echo $item_id; ?>" class="widefat code edit-menu-item-classes" name="menu-item-classes[<?php echo $item_id; ?>]" value="<?php echo esc_attr( implode(' ', $item->classes ) ); ?>" />
 					</label>
 				</p>
 				<p class="field-xfn description description-thin">
diff --git a/wp-includes/nav-menu-template.php b/wp-includes/nav-menu-template.php
index 4f6e8536e..21236e4f0 100644
--- a/wp-includes/nav-menu-template.php
+++ b/wp-includes/nav-menu-template.php
@@ -72,7 +72,7 @@ class Walker_Nav_Menu extends Walker {
 
 		$classes = array( 'menu-item', 'menu-item-type-' . $item->type );
 		if ( !empty($item->classes) )
-			$classes = array_merge($classes, explode(' ', $item->classes) );
+			$classes = array_merge( $classes, $item->classes );
 
 		if ( 'custom' != $item->type ) {
 			$classes[] = 'menu-item-object-' . $item->object;
@@ -170,7 +170,7 @@ class Walker_Nav_Menu_Checklist extends Walker_Nav_Menu  {
 		$output .= '<input type="hidden" class="menu-item-target" name="menu-item[' . $possible_object_id . '][menu-item-target]" value="'. esc_attr( $item->target ) .'" />';
 		$output .= '<input type="hidden" class="menu-item-attr_title" name="menu-item[' . $possible_object_id . '][menu-item-attr_title]" value="'. esc_attr( $item->attr_title ) .'" />';
 		$output .= '<input type="hidden" class="menu-item-description" name="menu-item[' . $possible_object_id . '][menu-item-description]" value="'. esc_attr( $item->description ) .'" />';
-		$output .= '<input type="hidden" class="menu-item-classes" name="menu-item[' . $possible_object_id . '][menu-item-classes]" value="'. esc_attr( $item->classes ) .'" />';
+		$output .= '<input type="hidden" class="menu-item-classes" name="menu-item[' . $possible_object_id . '][menu-item-classes]" value="'. esc_attr( implode( ' ', $item->classes ) ) .'" />';
 		$output .= '<input type="hidden" class="menu-item-xfn" name="menu-item[' . $possible_object_id . '][menu-item-xfn]" value="'. esc_attr( $item->xfn ) .'" />';
 	}
 }
@@ -338,7 +338,7 @@ function _wp_menu_item_classes_by_context( &$menu_items = array() ) {
 				( 'taxonomy' == $menu_item->type && ( $wp_query->is_category || $wp_query->is_tag || $wp_query->is_tax ) )
 			)
 		) {
-			$menu_items[$key]->classes = trim( $menu_items[$key]->classes . ' ' . 'current-menu-item' );
+			$menu_items[$key]->classes[] = 'current-menu-item';
 			$active_parent_item_ids[] = (int) $menu_item->menu_item_parent;
 			$active_parent_object_ids[] = (int) $menu_item->post_parent;
 			$active_object = $menu_item->object;
@@ -348,9 +348,9 @@ function _wp_menu_item_classes_by_context( &$menu_items = array() ) {
 			$current_url = ( is_ssl() ? 'https://' : 'http://' ) . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];
 			$item_url = strpos( $menu_item->url, '#' ) ? substr( $menu_item->url, 0, strpos( $menu_item->url, '#' ) ) : $menu_item->url;
 			if ( $item_url == $current_url ) {
-				$menu_items[$key]->classes = trim( $menu_item->classes . ' ' . 'current-menu-item' );
+				$menu_items[$key]->classes[] = 'current-menu-item';
 				if ( untrailingslashit($current_url) == home_url() )
-					$menu_items[$key]->classes .= ' menu-item-home';
+					$menu_items[$key]->classes[] = 'menu-item-home';
 				$active_parent_item_ids[] = (int) $menu_item->menu_item_parent;
 				$active_parent_object_ids[] = (int) $menu_item->post_parent;
 				$active_object = $menu_item->object;
@@ -359,7 +359,7 @@ function _wp_menu_item_classes_by_context( &$menu_items = array() ) {
 		
 		// back-compat with wp_page_menu: add "current_page_parent" to static home page link for any non-page query
 		if ( ! empty( $home_page_id ) && 'post_type' == $menu_item->type && empty( $wp_query->is_page ) && $home_page_id == $menu_item->object_id ) 
-			$menu_items[$key]->classes = trim( $menu_items[$key]->classes . ' ' . 'current_page_parent' );
+			$menu_items[$key]->classes[] = 'current_page_parent';
 	}
 
 	$active_parent_item_ids = array_filter( array_unique( $active_parent_item_ids ) );
@@ -374,11 +374,11 @@ function _wp_menu_item_classes_by_context( &$menu_items = array() ) {
 			is_post_type_hierarchical( $queried_object->post_type ) &&
 			in_array( $parent_item->object_id, $queried_object->ancestors )
 		)
-			$menu_items[$key]->classes = trim( $parent_item->classes . ' ' . 'current-' . $queried_object->post_type . '-ancestor current-menu-ancestor' );
+			$menu_items[$key]->classes[] = 'current-' . $queried_object->post_type . '-ancestor current-menu-ancestor';
 		if ( in_array( $parent_item->db_id, $active_parent_item_ids ) )
-			$menu_items[$key]->classes = trim( $parent_item->classes . ' ' . 'current-menu-parent' );
+			$menu_items[$key]->classes[] = 'current-menu-parent';
 		if ( in_array( $parent_item->object_id, $active_parent_object_ids ) )
-			$menu_items[$key]->classes = trim( $parent_item->classes . ' ' . 'current-' . $active_object . '-parent' );
+			$menu_items[$key]->classes[] = 'current-' . $active_object . '-parent';
 	}
 }
 
diff --git a/wp-includes/nav-menu.php b/wp-includes/nav-menu.php
index 1ec93f9f8..66424aa42 100644
--- a/wp-includes/nav-menu.php
+++ b/wp-includes/nav-menu.php
@@ -253,7 +253,6 @@ function wp_update_nav_menu_object( $menu_id = 0, $menu_data = array() ) {
  * @return int The menu item's database ID or WP_Error object on failure.
  */
 function wp_update_nav_menu_item( $menu_id = 0, $menu_item_db_id = 0, $menu_item_data = array() ) {
-
 	$menu_id = (int) $menu_id;
 	$menu_item_db_id = (int) $menu_item_db_id;
 
@@ -370,9 +369,9 @@ function wp_update_nav_menu_item( $menu_id = 0, $menu_item_db_id = 0, $menu_item
 		update_post_meta( $menu_item_db_id, '_menu_item_object_id', (int) $args['menu-item-object-id'] );
 		update_post_meta( $menu_item_db_id, '_menu_item_object', sanitize_key($args['menu-item-object']) );
 		update_post_meta( $menu_item_db_id, '_menu_item_target', sanitize_key($args['menu-item-target']) );
-		// @todo sanitize_html_classes()
-		foreach( array( 'menu-item-classes', 'menu-item-xfn') as $arg )
-			$args[$arg] = implode( ' ', array_map( 'sanitize_html_class', explode( ' ', $args[$arg] ) ) );
+		
+		$args['menu-item-classes'] = array_map( 'sanitize_html_class', explode( ' ', $args['menu-item-classes'] ) );
+		$args['menu-item-xfn'] = implode( ' ', array_map( 'sanitize_html_class', explode( ' ', $args['menu-item-xfn'] ) ) );
 		update_post_meta( $menu_item_db_id, '_menu_item_classes', $args['menu-item-classes'] );
 		update_post_meta( $menu_item_db_id, '_menu_item_xfn', $args['menu-item-xfn'] );
 
@@ -533,7 +532,7 @@ function wp_get_nav_menu_items( $menu, $args = array() ) {
  * - title:		The title of this menu item.
  * - target: 		The target attribute of the link element for this menu item.
  * - attr_title:	The title attribute of the link element for this menu item.
- * - classes:		The class attribute value for the link element of this menu item.
+ * - classes:		The array of class attribute values for the link element of this menu item.
  * - xfn:		The XFN relationship expressed in the link of this menu item.
  * - description:	The description of this menu item.
  *
@@ -580,7 +579,7 @@ function wp_setup_nav_menu_item( $menu_item ) {
 			$menu_item->attr_title = apply_filters( 'nav_menu_attr_title', $menu_item->post_excerpt );
 			$menu_item->description = apply_filters( 'nav_menu_description', $menu_item->post_content );
 
-			$menu_item->classes = get_post_meta( $menu_item->ID, '_menu_item_classes', true );
+			$menu_item->classes = (array) get_post_meta( $menu_item->ID, '_menu_item_classes', true );
 			$menu_item->xfn = get_post_meta( $menu_item->ID, '_menu_item_xfn', true );
 		} else {
 			$menu_item->db_id = 0;
@@ -598,7 +597,7 @@ function wp_setup_nav_menu_item( $menu_item ) {
 
 			$menu_item->attr_title = apply_filters( 'nav_menu_attr_title', $menu_item->post_excerpt );
 			$menu_item->description = apply_filters( 'nav_menu_description', $menu_item->post_content );
-			$menu_item->classes = '';
+			$menu_item->classes = array();
 			$menu_item->xfn = '';
 		}
 	} elseif ( isset( $menu_item->taxonomy ) ) {
@@ -618,7 +617,7 @@ function wp_setup_nav_menu_item( $menu_item ) {
 		$menu_item->target = '';
 		$menu_item->attr_title = '';
 		$menu_item->description = get_term_field( 'description', $menu_item->term_id, $menu_item->taxonomy );
-		$menu_item->classes = '';
+		$menu_item->classes = array();
 		$menu_item->xfn = '';
 
 	}