diff --git a/fastJSON/JsonSerializer.cs b/fastJSON/JsonSerializer.cs
index ea13fd0..d6a3587 100644
--- a/fastJSON/JsonSerializer.cs
+++ b/fastJSON/JsonSerializer.cs
@@ -238,7 +238,7 @@ private void WriteCustom(object obj)
         {
             Reflection.Serialize s;
             Reflection.Instance._customSerializer.TryGetValue(obj.GetType(), out s);
-            WriteStringFast(s(obj));
+            _output.Append(s(obj));
         }
 
         private void WriteEnum(Enum e)