LOG: failed to commit transaction_isolation

PostgreSQL 8.0.0beta2 (CVS)
Solaris 9

I've noticed the following message in the server's logs:

LOG: failed to commit transaction_isolation

The log message appears if I issue a SET TRANSACTION ISOLATION LEVEL
and later do a ROLLBACK.  For example:

BEGIN;
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;  -- or READ COMMITTED
SELECT VERSION();
ROLLBACK;

Why the log message?  Is this by design or is it a bug?

--
Michael Fuhr
http://www.fuhr.org/~mfuhr/