Courier MYSQL_QUOTA_FIELD isn't working

In /etc/courier/authmysqlrc, I have

MYSQL_QUOTA_FIELD CONCAT(quota, 'S')

But connecting to the account via RoundCube or Thunderbird with the Display Quota plugin shows an unlimited/unknown quota. In the quota field, I have entered 1000000, and this table/row is otherwise working perfectly for authentication.

I enabled mysql logging and checked the query log for the query that courier is executing for auth. When I execute it myself it works fine and correctly returns "1000000s" for the quota.

I'm using Ubuntu Server 12.04. Any ideas?