diff --git a/lib/compat/wordpress-6.0/class-wp-theme-json-6-0.php b/lib/compat/wordpress-6.0/class-wp-theme-json-6-0.php
index 878ac30c1b2c7d..07820ccbd1dbe2 100644
--- a/lib/compat/wordpress-6.0/class-wp-theme-json-6-0.php
+++ b/lib/compat/wordpress-6.0/class-wp-theme-json-6-0.php
@@ -592,7 +592,7 @@ public function get_data() {
 				}
 				$flattened_preset = array();
 				foreach ( $items as $slug => $value ) {
-					$flattened_preset[] = array_merge( array( 'slug' => $slug ), $value );
+					$flattened_preset[] = array_merge( array( 'slug' => (string) $slug ), $value );
 				}
 				_wp_array_set( $output, $path, $flattened_preset );
 			}