`for in String` runs without throwing Error

forinstring.js

var _keys = function(obj){
  //if(Object.keys) return Object.keys(obj);
  var keys = [];
  for(var k in obj){
    if(obj.hasOwnProperty(k)) keys.push(k);
  }
  return keys;
};

_keys('foo'); // [0, 1, 2]

forinstring2.js

var _keys = function(obj){
  //if(Object.keys) return Object.keys(obj);
  if (typeof obj != 'object' && typeof obj != 'function') {
      throw new TypeError('-');
  }
  var keys = [];
  for(var k in obj){
    if(obj.hasOwnProperty(k)) keys.push(k);
  }
  return keys;
};

try {
  alert(_keys('foo'));
} catch (err) {
  alert(err); // TypeError: -
}

run in jsFiddle