diff --git a/wp-includes/theme.php b/wp-includes/theme.php
index dae7ed7e4c..7d1669caf1 100644
--- a/wp-includes/theme.php
+++ b/wp-includes/theme.php
@@ -966,23 +966,31 @@ function validate_theme_requirements( $stylesheet ) {
  * Retrieves all theme modifications.
  *
  * @since 3.1.0
+ * @since 5.9.0 The return value is always an array.
  *
- * @return array|void Theme modifications.
+ * @return array Theme modifications.
  */
 function get_theme_mods() {
 	$theme_slug = get_option( 'stylesheet' );
 	$mods       = get_option( "theme_mods_$theme_slug" );
+
 	if ( false === $mods ) {
 		$theme_name = get_option( 'current_theme' );
 		if ( false === $theme_name ) {
 			$theme_name = wp_get_theme()->get( 'Name' );
 		}
+
 		$mods = get_option( "mods_$theme_name" ); // Deprecated location.
 		if ( is_admin() && false !== $mods ) {
 			update_option( "theme_mods_$theme_slug", $mods );
 			delete_option( "mods_$theme_name" );
 		}
 	}
+
+	if ( ! is_array( $mods ) ) {
+		$mods = array();
+	}
+
 	return $mods;
 }
 
diff --git a/wp-includes/version.php b/wp-includes/version.php
index f456c53adc..152d26baaf 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51523';
+$wp_version = '5.9-alpha-51524';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.