can't pass FALSE to db_update

This code causes an SQL error:

          $query = db_update('node_type')
            ->fields(array(
              'custom' => TRUE,
              'locked' => FALSE,
            ))          
            ->condition('type', 'article')
            ->execute();

The error message is:

PDOException: SQLSTATE[HY000]: General error: 1366 Incorrect integer value: '' for column 'locked' at row 1: UPDATE {node_type} SET custom=:db_update_placeholder_0, locked=:db_update_placeholder_1 WHERE (type = :db_condition_placeholder_0) ; Array ( [:db_update_placeholder_0] => 1 [:db_update_placeholder_1] => [:db_condition_placeholder_0] => article )  in field_convert_select() (line 37 of /Users/joachim/Sites/_sandbox/field_convert-HEAD/field_convert.admin.inc).

It appears the value of FALSE gets turned into an empty string.

Are there cases where this is the desired behaviour? If not, I think being able to pass FALSE and TRUE makes for more legible code.