ftell mismatch with expected position

I am getting the following warning when our program runs pg_dump.exe and
the output is in custom format and send to standard out which is connected
to a pipe (Windows platform).

pg_dump: [custom archiver] WARNING: ftell mismatch with expected position
-- ftell used

The output of pg_dump is received by another process that copies it (and
some other data) into a zip file. When I extract the db backup from the zip
the backup is fine and I can restore it.

I have both tried the pg_dump.exe from the 9.5 alpha and a stable 9.4 both
generate this warning.
When I change the format to plain the warning goes away.
When I call pg_dump from the commandline and let it send to standard out
and redirect it to a file the warning does not occur but it does occur when
I redirect to nul.

Is there a way to prevent this warning? I would think pg_dump shouldn't
call ftell on a pipe.

Regards, Eelke