diff --git a/src/InstanceSerialization/PHPDocType.php b/src/InstanceSerialization/PHPDocType.php
index 0cb730a..9514471 100644
--- a/src/InstanceSerialization/PHPDocType.php
+++ b/src/InstanceSerialization/PHPDocType.php
@@ -48,7 +48,7 @@ protected static function removeIfStartsWith(string &$haystack, $needle): bool {
 
   protected static function parseImpl(string &$str): ?PHPDocType {
     if (self::removeIfStartsWith($str, "?")) {
-      $str = "null|({$str})";
+      $str = "null|{$str}";
     }
 
     $res = InstanceType::parse($str) ?: