diff --git a/lib/ClassBuilder.js b/lib/ClassBuilder.js
index 6d94c36..c5c0a4c 100644
--- a/lib/ClassBuilder.js
+++ b/lib/ClassBuilder.js
@@ -822,7 +822,7 @@ exports.prototype.createCtor = function( cname, abstract_methods, members )
         new_class = this.createAbstractCtor( cname );
     }
 
-    new_class[ _priv ] = {};
+    util.defineSecureProp( new_class, _priv, {} );
     return new_class;
 }
 
@@ -1314,7 +1314,7 @@ function initInstance( instance )
 
     // initialize our *own* private metadata store; do not use the
     // prototype's
-    instance[ _priv ] = {};
+    util.defineSecureProp( instance, _priv, {} );
 
     // add the visibility objects to the data object for this class instance
     instance[ _priv ].vis = new prot();