diff --git a/lib/extensions.js b/lib/extensions.js
index b28ed09c..505bdf51 100644
--- a/lib/extensions.js
+++ b/lib/extensions.js
@@ -257,10 +257,12 @@ module.exports = function(realmConstructor) {
                 permissions: '__Permission[]'
             }
         });
-        Object.defineProperty(realmConstructor, 'Permissions', {
-            value: permissionsSchema,
-            configurable: false
-        });
+        if (!realmConstructor.Permissions) {
+            Object.defineProperty(realmConstructor, 'Permissions', {
+                value: permissionsSchema,
+                configurable: false
+            });
+        }
     }
 
     // TODO: Remove this now useless object.