InfoPath form linked to K2: issue with <my:SN> element

I have a workflow process running using an InfoPath form and K2. New forms are generated dynamically using a batch process that populates an xml file and then uploads it to a SharePoint library.

Sometimes (but not always) when the user tries to submit the form to K2, an error will be thrown complaining about an "invalid serial number".

If I go back into the xml and remove all whitespace between the tags, as in "<my:SN></my:SN>" and save back to the form library, the error disappears and the form submits. (I can also replace the 2 tags with a single "<my:SN />" and that also fixes it.)

Anyone have any idea why this error occurs?

Thanks,
Jim