Post SSO hidden form session variable display garbled on server

Hello everyone,

Use this irule to create a session variable.

when ACCESS_ACL_ALLOWED {

    ACCESS::session data set session.mylab.sso.returnuri [HTTP::uri]

}

Set up SSO profile hidden form

returnURL %{session.mylab.sso.returnuri}

Why returnURL is garbled displayed on the web application debug log?

thanks.