'use strict';
  
  var value = require('./valid-value')
  
    , propertyIsEnumerable = Object.prototype.propertyIsEnumerable;
  
  module.exports = function (obj) {
  	var i;
  	value(obj);
  	for (i in obj) {
  		if (propertyIsEnumerable.call(obj, i)) return i;
  	}
  	return null;
  };