diff --git a/wp-includes/theme.php b/wp-includes/theme.php
index 89eab5f790..6909d0fe2e 100644
--- a/wp-includes/theme.php
+++ b/wp-includes/theme.php
@@ -690,7 +690,7 @@ function switch_theme( $stylesheet ) {
 	}
 
 	if ( is_array( $_sidebars_widgets ) ) {
-		set_theme_mod( 'sidebars_widgets', $_sidebars_widgets );
+		set_theme_mod( 'sidebars_widgets', array( 'time' => time(), 'data' => $_sidebars_widgets ) );
 	}
 
 	$nav_menu_locations = get_theme_mod( 'nav_menu_locations' );
diff --git a/wp-includes/version.php b/wp-includes/version.php
index a23abcd8a8..47064c35d3 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9-alpha-41672';
+$wp_version = '4.9-alpha-41673';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.
diff --git a/wp-includes/widgets.php b/wp-includes/widgets.php
index 37e8a01c52..6f5954d322 100644
--- a/wp-includes/widgets.php
+++ b/wp-includes/widgets.php
@@ -1286,6 +1286,7 @@ function wp_map_sidebars_widgets( $existing_sidebars_widgets ) {
 
 	// Sidebars_widgets settings from when this theme was previously active.
 	$old_sidebars_widgets = get_theme_mod( 'sidebars_widgets' );
+	$old_sidebars_widgets = $old_sidebars_widgets['data'];
 
 	if ( is_array( $old_sidebars_widgets ) ) {