diff --git a/src/cc/nnproject/json/JSONObject.java b/src/cc/nnproject/json/JSONObject.java
index 91a6264..161c6b5 100644
--- a/src/cc/nnproject/json/JSONObject.java
+++ b/src/cc/nnproject/json/JSONObject.java
@@ -220,11 +220,11 @@ public void put(String name, Object obj) {
 	}
 	
 	public void put(String name, AbstractJSON json) {
-		table.put(name, json);
+		table.put(name, json == null ? JSON.json_null : json);
 	}
 	
 	public void put(String name, String s) {
-		table.put(name, s);
+		table.put(name, s == null ? JSON.json_null : s);
 	}
 
 	public void put(String name, int i) {