Thank you for your reply Ruselw,
I change the API and it works fine now. Its funny, even SDK sample “BAMFROMEXPRESSION” throwing same exception. I am not sure why.
One more question, Would it be possible to handle duplicates in BAM? so if i receive duplicate invoices from same vendor, then i want to show that in the BAM screen. For example, if I use Invoice# as ActivityID, then BAM will fail when i receive dups. Would it be possible to handle this failure and show as dups?