APM httpagent401 in clientless mode?

I've noticed that when using clientless mode the logging statement for username is blank. When using the normal Login Page agent, this information is populated once the login button is submitted by the client.

Here is my iRule configuration:

when HTTP_REQUEST {
    if { ([HTTP::header exists "Authorization"]) && ([HTTP::cookie value MRHSession] == "") } {
        switch [IP::addr [IP::client_addr] mask 255.255.0.0] {
            "10.3.0.0" {
                HTTP::header insert "clientless-mode" 1
                set clientless_mode 1
            }
        }
    }
}

when ACCESS_SESSION_STARTED {
    set sid [ACCESS::session data get "session.user.sessionid"]
    if { [info exists clientless_mode] } {
        log -noname accesscontrol.local1.notice "01490000:7: $sid: ---New Clientless Session---"
        ACCESS::session data set session.logon.last.username [HTTP::username]
        ACCESS::session data set -secure session.logon.last.password [HTTP::password]
        ACCESS::session data set session.inactivity_timeout  120
        ACCESS::session data set session.max_session_timeout 120
        unset clientless_mode
    }
}

/var/log/apm Entry

notice apd[13742]: 01490010:5: ae67c781: Username ''

Any ideas on how to get the Username log statement populated?