this[View.prototype.i18n.defaultLocale][key] is undefined

view_i18n.js

/**
*view_i18n.js
*this[View.prototype.i18n.defaultLocale][key] is undefined 
*Line 56  

*
* key is the key used to lookup the value in i18n_XX.js file
* locale is which locale to use. will be appended in i18n_[locale].js
* if local does not exist, use defaultLocale
* if key is not found, use defaultLocale's values
*/
View.prototype.i18n.getString = function(key,locale) {
  // if specified locale does not exist, use default locale
  if (View.prototype.i18n.supportedLocales.indexOf(locale) == -1) {
    locale = View.prototype.i18n.defaultLocale;
  }
  if (this[locale][key] !== undefined) {
    return this[locale][key].value;
  } else {
    return this[View.prototype.i18n.defaultLocale][key].value;    // <== line 56 in source.
  }
};