pg 8.1.2 ERROR: direct correlated subquery unsupported as initplan

Just encountered this error on an 8.1.2 cluster:

LOG:  statement: PREPARE dbdpg_5 AS SELECT * FROM foo WHERE   visitnum = $1 and  (scheduledtime >= $2 OR scheduledtime
ISNULL) AND  (scheduledtime < $3 OR scheduledtime IS NULL)  UNION SELECT * FROM bar WHERE   visitnum = $4  ORDER BY
scheduledtime
LOG:  statement: <BIND>
LOG:  statement: EXECUTE <unnamed>  [PREPARE:  SELECT * FROM foo WHERE   visitnum = $1 and  (scheduledtime >= $2 OR
scheduledtimeIS NULL) AND  (scheduledtime < $3 OR scheduledtime IS NULL)  UNION SELECT * FROM bar WHERE   visitnum = $4
ORDER BY scheduledtime] 
ERROR:  direct correlated subquery unsupported as initplan

A run of 'analyze' seems to have made the error go away.

Here's our version:

PostgreSQL 8.1.2 on ia64-unknown-linux-gnu, compiled by GCC gcc (GCC) 3.4.3 20050227 (Red Hat 3.4.3-22.1)

Any clues?

Thanks,
Ed