diff --git a/mozilla/js/narcissus/jsexec.js b/mozilla/js/narcissus/jsexec.js
index 700b96b26d4..04111222582 100644
--- a/mozilla/js/narcissus/jsexec.js
+++ b/mozilla/js/narcissus/jsexec.js
@@ -740,8 +740,10 @@ function Activation(f, a) {
 
 
 // Null Activation.prototype's proto slot so that Object.prototype.* does not
-// pollute the scope of heavyweight functions.
+// pollute the scope of heavyweight functions.  Also delete its 'constructor'
+// property so that id doesn't pollute function scopes.
 Activation.prototype.__proto__ = null;
+delete Activation.prototype.constructor;
 
 function FunctionObject(node, scope) {
     this.node = node;