diff --git a/Implab/JSON/JSONWriter.cs b/Implab/JSON/JSONWriter.cs
--- a/Implab/JSON/JSONWriter.cs
+++ b/Implab/JSON/JSONWriter.cs
@@ -197,8 +197,10 @@ namespace Implab.JSON {
  
 
         void Write(string value) {
-            if (value == null)
+            if (value == null) {
                 m_writer.Write("null");
+                return;
+            }
 
             var chars = value.ToCharArray();
             m_writer.Write('"');