MySQL query not using index

I have a MySQL query that is running very slowly because it's not using an index on the join key. What is wrong with my table structure that the optimizer is not using the index?

mysql> EXPLAIN SELECT *  FROM indicator_performance_averages
LEFT OUTER JOIN `prof` ON (`prof`.`symbol` = indicator_performance_averages.`symbol`) 
WHERE (indicator_performance_averages.`symbol` = 'ZCN13');
+----+-------------+--------------------------------+-------+---------------+------------+---------+-------+---------+-------+
| id | select_type | table                          | type  | possible_keys | key        | key_len | ref   | rows    | Extra |
+----+-------------+--------------------------------+-------+---------------+------------+---------+-------+---------+-------+
|  1 | SIMPLE      | indicator_performance_averages | const | idx_symbol    | idx_symbol | 98      | const |       1 |       |
|  1 | SIMPLE      | prof                           | ALL   | NULL          | NULL       | NULL    | NULL  | 1102075 |       |
+----+-------------+--------------------------------+-------+---------------+------------+---------+-------+---------+-------+

mysql> DESCRIBE indicator_performance_averages;
+--------------------+------------------+------+-----+---------+----------------+
| Field              | Type             | Null | Key | Default | Extra          |
+--------------------+------------------+------+-----+---------+----------------+
| id                 | int(11) unsigned | NO   | PRI | NULL    | auto_increment |
| symbol             | varchar(32)      | NO   | UNI | NULL    |                |
| date               | date             | YES  |     | NULL    |                |
| last_update        | timestamp        | YES  |     | NULL    |                |
+--------------------+------------------+------+-----+---------+----------------+

mysql> DESCRIBE prof;
+---------------+---------------------+------+-----+---------------------+----------------+
| Field         | Type                | Null | Key | Default             | Extra          |
+---------------+---------------------+------+-----+---------------------+----------------+
| id            | bigint(20) unsigned | NO   | PRI | NULL                | auto_increment |
| symbol        | varchar(32)         | NO   | UNI | NULL                |                |
| name          | varchar(128)        | NO   |     | NULL                |                |
| lastupdate    | datetime            | NO   |     | 0000-00-00 00:00:00 |                |
+---------------+---------------------+------+-----+---------------------+----------------+

EDIT: Adding SHOW CREATE TABLE for both tables:

CREATE TABLE `indicator_performance_averages` (
  `id` int(11) unsigned NOT NULL auto_increment,
  `symbol` varchar(32) character set utf8 NOT NULL,
  `date` date default NULL,
  `last_update` timestamp NULL default NULL,
  PRIMARY KEY  (`id`),
  UNIQUE KEY `idx_symbol` (`symbol`)
) ENGINE=InnoDB AUTO_INCREMENT=6719 DEFAULT CHARSET=latin1

CREATE TABLE `prof` (
  `id` bigint(20) unsigned NOT NULL auto_increment,
  `symbol` varchar(32) NOT NULL,
  `name` varchar(128) NOT NULL,
  `lastupdate` datetime NOT NULL default '0000-00-00 00:00:00',
  PRIMARY KEY  (`id`),
  UNIQUE KEY `id_UNIQUE` (`id`),
  UNIQUE KEY `IDX_symbol` (`symbol`),
) ENGINE=InnoDB AUTO_INCREMENT=37736071 DEFAULT CHARSET=latin1