Why does this give a TypeError: cannot convert undefined or null to object?

ObjectCopy.js

"use strict";

function copy(proto, changes) {
    if (changes === undefined) return Object.create(proto);
    var props = Object.keys(changes).map(function(attr) {
        var single = {};
        single[attr] = {value: changes[attr], writable: false, enumerable: true};
        return single;
    });
    return Object.create(proto, Object.assign.apply(props));
}

var c = {r: 0, g: 0, b: 0};
copy(c, {r: 1, b: 1});