column seems to be missing from map table: "Column not found: 1054 Unknown column 'destid2' in 'field list':"

I'm getting this error for each row I import:

SQLSTATE[42S22]: Column not found: 1054 Unknown column 'destid2' in 'field list': INSERT INTO {migrate_map_greavesproduct} (sourceid1, sourceid2, needs_update, destid1, destid2) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4_0); Array ( [:db_insert_placeholder_0] => 1 [:db_insert_placeholder_1] => 11300033020 [:db_insert_placeholder_2] => 0 [:db_insert_placeholder_3] => 22 [:db_insert_placeholder_4_0] => 22 ) (sites/all/modules/contrib/migrate/plugins/sources/sqlmap.inc:317)

This is a column that's missing from the map table for this particular migration. My map is declared thus:

    $this->map = new MigrateSQLMap($this->machineName,
      array(
        'product_id' => array(
          'type' => 'varchar',
          'length' => 255,
          'not null' => TRUE,
          'description' => "Product mapping ID from the CSV",
        ),
        'gs_ref' => array(
          'type' => 'varchar',
          'length' => 255,
          'not null' => TRUE,
          'description' => "The product code",
        ),
      ),
      // Need to specify the entity type.
      MigrateDestinationEntityAPI::getKeySchema('commerce_product')
    );

...which all looks fine to me :)

I've checked the return value of MigrateDestinationEntityAPI::getKeySchema('commerce_product'), and it produces two keys, the product ID and revision ID. But there's only one destination column in the map table.

I've looked at other destination classes in Migrate, and none that I can see return more than 1 key in their getKeySchema().

Might this be a bug in Migrate that we're hitting?