Using the API for searching on a custom datetime field with '<=' operator throws error

Suppose you have a custom datetime field on contacts, and you want to search all contacts that have a date in this field that is earlier than some given date, like e.g.

    $result = civicrm_api3('Contact', 'get', array(
      'sequential' => 1,
      'custom_2' => array('<=' => "2015-08-31"),
    ));

Then you get this unrelated error message:

    {
        "is_error": 1,
        "error_message": "The field 'custom_2' has no associated option list."
    }

I am aware of the fact that the custom fields you provide to the api are currently ignored (CRM-16036), but this issue seems to be unrelated.
UPDATE: searching on custom fields does work for contacts.

I will upload a unit test in a minute.