diff --git a/wp-includes/customize/class-wp-customize-custom-css-setting.php b/wp-includes/customize/class-wp-customize-custom-css-setting.php
index 0e6132f339..aab0e47530 100644
--- a/wp-includes/customize/class-wp-customize-custom-css-setting.php
+++ b/wp-includes/customize/class-wp-customize-custom-css-setting.php
@@ -197,9 +197,10 @@ final class WP_Customize_Custom_CSS_Setting extends WP_Customize_Setting {
 			)
 		);
 
-		if ( $r instanceof WP_Error ) {
+		if ( is_wp_error( $r ) ) {
 			return false;
 		}
+
 		$post_id = $r->ID;
 
 		// Cache post ID in theme mod for performance to avoid additional DB query.
diff --git a/wp-includes/version.php b/wp-includes/version.php
index fba2a1d5d9..62f039a94d 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.9-alpha-60213';
+$wp_version = '6.9-alpha-60215';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.