cannot implicitly convert type 'System.Int64' to 'System.String'

Hi.

I'm try to use a distinguished filed as a file name in a send port.

I use message assigment in orch, and i have this text in this message assigment.

MyMessage(FILE.ReceiveFileName) = MyMessage.and this distinguished filed name.

And then i get this error: cannot implicitly convert type 'System.Int64' to 'System.String'

I have to used this distinguished filed name as a file name becouse my partner can't read files whit %MessageID%..  This filename be to long for him.

regards

Daniel