TypeError: 'set' on proxy: trap returned falsish for property

Hello awesome polly! Got an error not sure if it is me but it only happens when I use strict;. I am not adding a new path just editing existing path. If it is a bug lets see if we can fix it. If it is me slap me in the face but tell me what I am doing wrong please.

error: TypeError: 'set' on proxy: trap returned falsish for property 'port'

Example

'use strict'; // this makes the error throw

function ObserveObject(object, callback) {
    function createProxy(prefix, object) {
        return new Proxy(object, {
            set: function(target, property, value) {
                target[property] = value;
                callback(prefix + property, value);
            },
            get: function(target, property) {
                var value = target[property];

                if (isObject(value)) return createProxy(prefix + property + '.', value); 
                else return value; 
            }
        });
    }

    return createProxy('', object);
}

function isObject(value) {
    if (value === null || value === undefined) return false;
    else return value.constructor === Object;
}

var o = {
    hello: 'blue',
    app: {
        env: {
            port: 7777,
            la: 1
        }
    },
    more: 'stuff'
};

var m = ObserveObject (o, function(path, value) {
    console.log(path);
    console.log(value);
});

m.app.env.port = 8888;
console.log(m);