Database error in flag_flag_access_multiple when trying to view bookmarks

One my test site, I enabled the module Flag and I'm testing out the "bookmarks" application, as a demo for getting used to the module. BTW I'm using Postgres as my database.

I have 1 node (of custom the content type "car") and a test user that bookmarked it. Now as that user I try to look at my bookmarks. Nothing fancy. I get a fatal error:

PDOException: SQLSTATE[22P02]: Invalid text representation: 7 ERROR: invalid input syntax for integer: "unflag": SELECT n.nid AS nid, n.uid AS uid FROM {node} n WHERE (nid IN (:db_condition_placeholder_0)) AND (type IN (:db_condition_placeholder_1, :db_condition_placeholder_2)) ; Array ( [:db_condition_placeholder_0] => unflag [:db_condition_placeholder_1] => car [:db_condition_placeholder_2] => article ) in flag_flag_access_multiple() (line 954 of /var/www/html/dev/cambiosale/sites/all/modules/flag/flag.module).

The code it chokes on is this statement:

    $result = db_select('node', 'n')
      ->fields('n', array('nid', 'uid'))
      ->condition('nid', $content_ids, 'IN')
      ->condition('type', $flag->types, 'IN')
      ->execute();

I added code to dump a few variables as JSON, and I get this:

content_ids : {"1":"unflag"}
flag->types: ["car","article"]

The problem as I see it, in in the parameter $content_ids. I think the db_select statement expects it to be an array of integers, but what it gets instead is a sparse array of strings. Likely the intention is to use the array_keys for that parameter. (Note: the single node that I have has the node id 1.)

This seems to be so fundamentally broken... I have no idea why it appears I'm the only one having this problem.

I also made a stack dump (depth 10), see attachment. I took care not to add any lines, so the line numbers are the same as in the original. (The function parameters are dumped in JSON format, using json_encode(), which dumps an object as an associative array. Dumps are truncated, when too long, per parameter.)

Note: I tried 7.x-2.x-dev but it's the same problem, only the line number changes.