Prototype property returned on access despite later override to null

[natevw]

var util = require('util');

function Base() {}
Base.prototype.foo = function () {
    console.log("!!!");
};

var obj2 = new Base();
obj2.foo = null;
console.log("???", obj2.foo);

function Subclass() {}
util.inherits(Subclass, Base);
Subclass.prototype.foo = (void 0);

var obj = new Subclass();
obj.foo();

Expected:

??? null
TypeError: Property 'foo' of object # is not a function

Actual:

??? function () { [compiled code] }
!!!

This was masking an issue where the HTTPS agent prototype's socket creation method was not actually getting overridden as expected, instead the HTTP agent method was just getting called!