*
       * https://github.com/typetools/checker-framework/issues/3030
       *
       * https://github.com/typetools/checker-framework/issues/3236
       */
      String key = (String) requireNonNull(e.nextElement());
      /*
       * requireNonNull is safe because the key came from propertyNames...
       *
       * ...except that it's possible for users to insert a string key with a non-string value, and