Problem about define receiver system filter

Hi,experts.

I have a problem about define receiver system filter.

In the Define Receiver Filter,I input the xpath like this ,/vpf:Msg/vpf:Header/vpf:Sender/@Id = /vpf:Msg/vpf:Header/vpf:ReceiverList/vpf:Receiver[./handover='P']/@Id.But when I run the step ,the B1if show me the error:Prefix "vpf" is not mapped to the namespace.If I delete the vpf,the filter always failed,all outbound are filtered.Anyone else can tell me how to sovled this problem?

Thanks a lot.

Regards.

lee