Bug in parser?

I have a program that executes the following command:

UPDATE ActiveUser SET status=-1
WHERE (datetime('today'::date, 'now'::time) - lastrequest) >
timespan('10 minutes'::reltime)

("lastrequest" is of type "datetime")

When executed via ExecCommandOk(), I get the following error message
back from ErrorMessage():

parser: parser error at or near ""

However, if I cut this command from the terminal window in which the
program is running (the command string is dumped to stdout just before
execution), and paste it in psql, it executes just fine.

Does this appear to be a bug in the libpq++ library? Am I using SQL
incorrectly here? Something else, maybe?

Thanks in advance.

Cheers,
Richard