diff --git a/src/main/org/codehaus/groovy/runtime/ConversionHandler.java b/src/main/org/codehaus/groovy/runtime/ConversionHandler.java
index 6791cb0..cbf69e7 100644
--- a/src/main/org/codehaus/groovy/runtime/ConversionHandler.java
+++ b/src/main/org/codehaus/groovy/runtime/ConversionHandler.java
@@ -93,7 +93,7 @@
             Object handle = handleCache.get(method);
             if (handle == null) {
                 handle = plugin.getInvokeSpecialHandle(method, proxy);
-                handleCache.put(method, handle);
+                handleCache.putIfAbsent(method, handle);
             }
             return plugin.invokeHandle(handle, args);
         }