MySQL, temporary on disk table on primary key retrieval?

Question

MySQL 5.5.24, Windows 7 64-bit.

I've InnoDB table with primary key defined on three columns:

CREATE TABLE `common_descriptions` (
  `object_id` mediumint(8) unsigned NOT NULL DEFAULT '0',
  `object_type` varchar(32) NOT NULL DEFAULT '',
  `description` mediumtext NOT NULL,
  `lang_code` char(2) NOT NULL DEFAULT '',
  `object` varchar(12) NOT NULL DEFAULT '',
  `object_holder` varchar(32) NOT NULL DEFAULT '',
  PRIMARY KEY (`object_id`,`lang_code`,`object`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

And I'm executing fairly simple query which (in my opinion) should use index to return rows

select SQL_NO_CACHE object_id, lang_code, object
from common_descriptions
limit 1

yet it creates 10 temporary tables of which 3 are on disk temporary tables (confirmed multiple times by monitoring show status like '%tmp%';). object column seems to trigger creation of temporary tables. When I replace object (varchar(12)) with description (mediumtext) temporary table is not created. explain extended of above query:

mysql> explain extended select SQL_NO_CACHE object_id, lang_code, object from co
mmon_descriptions limit 1\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: common_descriptions
         type: index
possible_keys: NULL
          key: PRIMARY
      key_len: 47
          ref: NULL
         rows: 49765
     filtered: 100.00
        Extra: Using index
1 row in set, 1 warning (0.00 sec)

Both tmp_table_size and max_heap_table_size are set to 16M.

After reading documentation on temporary tables (http://dev.mysql.com/doc/refman/5.1/en/internal-temporary-tables.html) I don't see any good explanation on why temporary tables would be created in this case. Why is it happening?