Hi ckreds,
It looks like you have specified a month value that Oracle doesn't recognize. You might want to verify that your selection criteria is using the right date format.
Take a look at the following URLs for a better definition of the error code.
http://www.dba-oracle.com/t_ora_01843_not_a_valid_month.htm
http://www.techonthenet.com/oracle/errors/ora01843.php
Hope this helps.
Regards,