APEX

I have a report region with a source that references both a text field and a select list that are on another region on the same page. I choose a date from the select list and then a number in the text field and press Enter to Submit. This is supposed to cause the region to display. But instead I keep getting the error message "ERR 1002 Unable to find item ID for item P81_TAG_PERIOD in application 100"
P81_TAG_PERIOD is the select list. It is named exactly the way it's spelled in the message and in the Source of the report region. So what could be going wrong here? Thanks.