Distributing tasks to portal users from external database

I have workflow 6.5.1 tasks that need to be distrubuted to users set up in MyWebmethods portal from an external database. I can log on as these users in the portal, but when I try to distribute a task, I get the following error.

7/12/06 [11:50:29.795 AM EDT] nullLogged by AuthenticationServer.0
[WKA.000.0019] Security API Tracing DEBUG:
[WKA.000.0052] French: getUser returns
user=null|full name=French DA|perm=63

7/12/06 [11:50:29.795 AM EDT] ERROR: Logged by AuthenticationServer.0
java.lang.Exception: [WKA.012.0007] WFUser object has NULL user name.

Has anyone run into this problem? If so, were you able to resolve it.