diff --git a/JSONObject.java b/JSONObject.java
index 399f093c1..d442aff31 100755
--- a/JSONObject.java
+++ b/JSONObject.java
@@ -1128,14 +1128,16 @@ public JSONObject put(String key, Object value) throws JSONException {
         }
         if (value != null) {
             testValidity(value);
-            pooled = (String)keyPool.get(key);
-            if (pooled == null) {
-                if (keyPool.size() >= keyPoolSize) {
+            synchronized (JSONObject.class) {
+              pooled = (String)keyPool.get(key);
+              if (pooled == null) {
+                  if (keyPool.size() >= keyPoolSize) {
                     keyPool = new HashMap(keyPoolSize);
-                }
-                keyPool.put(key, key);
-            } else {
-                key = pooled;
+                  }
+                  keyPool.put(key, key);
+              } else {
+                  key = pooled;
+              }
             }
             this.map.put(key, value);
         } else {