Persist lookup and a forwarding server

I have a guest running 11.6.0 HF6 with a forwarding virtual server defined as follows:

 

ltm virtual VSF_ise-1700 {
    description "SNAT forward for 1700"
    destination 0.0.0.0:mps-raft
    ip-protocol udp
    mask any
    profiles {
        ise_radius { }
        udp { }
    }
    rules {
        SNATfor1700
    }
    source 0.0.0.0/0
    translate-address disabled
    translate-port disabled
    vs-index 20
}

 

In my SNATfor1700 iRule, I am trying to look up the persistence record for a client based on the MAC address and the NAS IP in a Change-of-Authorization packet. The persistence records exist correctly, as I see them when I execute a "show ltm persistence persist-records" command in tmsh. However, this line in my iRule always returns an empty result:

 

  set persistInfo [persist lookup uie { $mac_up any pool }]

 

where $mac_up is the correct/verified key to the persistence record. Is there some trick to persist lookup that I am missing?

Thanks, Jen