db_update() fails on expression-only SQL

As I was going through #226876: DBTNG poll.module I noticed that a query I tried to rewrite with DBTNG failed:

Original

db_query("UPDATE {poll_choices} SET chvotes = chvotes + 1 WHERE chid = %d", $choice);

Rewrite:

db_update('poll_choices')
  ->expression('chvotes', 'chvotes + 1')
  ->condition('chid', $choice)
  ->execute();

The latter should be perfectly valid, but I get a PDO exception:
Invalid argument supplied for foreach() Warning query.inc 916 UpdateQuery->execute()
Invalid argument supplied for foreach() Warning query.inc 939 UpdateQuery->__toString()