Backend message type 0x50 arrived while idle

To all-
I am using Postgres version 6.5.1 in a multithreaded program which is
written in C. All inserts and updates into any tables are done within the
child process, and there are retrieves from tables both in the parent and
child processes. Additionally, the child process will insert into and
update a table, and the parent process will retrieve from the same table.
There are loops in both the parent and child processes, so every postgres
call may be made multiple times, with a significant amount of time (seconds
or minutes) between calls. Invariably, I get a message "backend message
type 0x50 arrived while idle" before the process completes, although the
point where I get this message can vary somewhat. Can anyone tell me what
this message means, why I am getting it, and/or how to avoid getting it in
the future?

Thanks in advance,
Phil