missing chunk number 0 for toast value when pg_dump??

hi, all:
I use pg_dump command to dump a specified  table:
pg_dump mydb -p 5555 -Uyahoo -t mytable -D -a > /export/dump.sql

but, I got this:
-----------------------
pg_dump: SQL command failed
pg_dump: Error message from server: ERROR:  missing chunk number 0 for
toast value 14319826
pg_dump: The command was: FETCH 100 FROM _pg_dump_cursor
-----------------------

the table have about 3,000,000 records, but from the dump file, I got
only 300,000 records with "INSERT INTO".

my postgresql version is :8.3.3.

thanks a lot.

regards,