diff --git a/src/wp-includes/widgets.php b/src/wp-includes/widgets.php
index 3db5fb8c2a..c79bff2e48 100644
--- a/src/wp-includes/widgets.php
+++ b/src/wp-includes/widgets.php
@@ -1294,6 +1294,9 @@ function wp_map_sidebars_widgets( $existing_sidebars_widgets ) {
 
 	if ( is_array( $old_sidebars_widgets ) ) {
 
+		// Remove empty sidebars, no need to map those.
+		$old_sidebars_widgets = array_filter( $old_sidebars_widgets );
+
 		// Only check sidebars that are empty or have not been mapped to yet.
 		foreach ( $new_sidebars_widgets as $new_sidebar => $new_widgets ) {
 			if ( array_key_exists( $new_sidebar, $old_sidebars_widgets ) && ! empty( $new_widgets ) ) {
diff --git a/tests/phpunit/tests/widgets.php b/tests/phpunit/tests/widgets.php
index c37dcc08a0..ed103d3008 100644
--- a/tests/phpunit/tests/widgets.php
+++ b/tests/phpunit/tests/widgets.php
@@ -709,7 +709,7 @@ class Tests_Widgets extends WP_UnitTestCase {
 		global $sidebars_widgets, $_wp_sidebars_widgets;
 
 		wp_widgets_init();
-		$this->register_sidebars( array( 'sidebar-1', 'sidebar-2', 'sidebar-3', 'wp_inactive_widgets' ) );
+		$this->register_sidebars( array( 'sidebar-1', 'sidebar-2', 'sidebar-3', 'sidebar-4', 'wp_inactive_widgets' ) );
 
 		// Test restoring sidebars widgets when previously activated.
 		set_theme_mod( 'sidebars_widgets', array(
@@ -718,6 +718,7 @@ class Tests_Widgets extends WP_UnitTestCase {
 				'sidebar-1' => array( 'tag_cloud-1' ),
 				'sidebar-2' => array(),
 				'sidebar-3' => array( 'unregistered_widget-1', 'text-1', 'media_image-1' ),
+				'sidebar-4' => null, // Should be converted to array.
 				'orphaned_widgets_1' => array( 'media_video-2' ),
 			),
 		) );