TypeError with Proxy class - TypeError: 'set' on proxy: trap returned truish for property

I am getting this fun error when using the Proxy class:

TypeError: 'set' on proxy: trap returned truish for property 'users' which exists in the proxy target as a non-configurable and non-writable data property with a different value

I have a library which creates proxy object properties recursively, where any non-primitive property is a Proxy object itself, etc etc.:

let mcProxy = function (target) {
  const mirrorCache = {};
  return new Proxy(target, {
    set: function (target, property, value, receiver) {
       if (mirrorCache[property]) {
          throw new Error(`property ${property} has already been set`);
       }
        mirrorCache[property] = true;
        Object.defineProperty(target, property, {
          writable: false,
          value: (value && typeof value === 'object') ? mcProxy(value) : value
        });
        return true;
    }
  });
};

exports.create = function (val) {
  val && assert.equal(typeof val, 'object', 'val must be an object');
  return mcProxy(val || {});
};

actual usage of the above library code:

//bash
$ npm install proxy-mcproxy

 // nodejs
 let McProxy = require('proxy-mcproxy');
 let val = McProxy.create();
 val.users = [];
 val.users = 3; // kaaaboom..error!

but when I set the users property the first time, I get the error in title of this question!

In my library code above, mirrorCache is a way to check if the property has been previously set. Want I want to do, is to throw an error, even if we aren't in strict mode, so mirrorCache appears to be necessary so that I do my own bookkeeping.

Perhaps there is a different or better way to achieve what I want to achieve? Here are my goals:

1. Throw an error even if not in strict mode.
2. Throw an error anytime the developer re-assigns a property. Each assigned property should be immutable.