diff --git a/lib/class-wp-theme-json-resolver-gutenberg.php b/lib/class-wp-theme-json-resolver-gutenberg.php
index e4787b468ab785..e433a104eac1fa 100644
--- a/lib/class-wp-theme-json-resolver-gutenberg.php
+++ b/lib/class-wp-theme-json-resolver-gutenberg.php
@@ -571,7 +571,8 @@ public static function get_merged_data( $origin = 'custom' ) {
 			_deprecated_argument( __FUNCTION__, '5.9.0' );
 		}
 
-		$result = static::get_core_data();
+		$result = new WP_Theme_JSON();
+		$result->merge( static::get_core_data() );
 		if ( 'default' === $origin ) {
 			$result->set_spacing_sizes();
 			return $result;