Creating ad-hoc task with array for custom data fails

The example for creating addhoc tasks in the 2.7 docs uses an array for custom data:

   $domination = new take_over_the_world();

   $domination->set_custom_data(array(

       'plansfortomorrownight' => 'The same thing we do every night, Pinky!'

   ));

   \core\task\manager::queue_adhoc_task($domination);

Actually doing this fails, because the custom data column is an object and so cannot be inserted into the DB.

See public static function record_from_adhoc_task($task):

$record->customdata = $task->get_custom_data();

And see adhoc_task::get_custom_data():

return json_decode($this->customdata);

json_decode will return an object, when what we want is the raw (encoded) string.