kamailio version 3.1.2

I am trying to setup dispatcher to use its failover feature. Here is dispatcher part of configure file:

loadmodule "dispatcher.so"

modparam("dispatcher", "db_url", DBURL)
modparam("dispatcher", "table_name", "dispatcher")
modparam("dispatcher", "ds_ping_interval", 30)
modparam("dispatcher", "ds_probing_threshhold", 10)
modparam("dispatcher", "ds_ping_reply_codes", "class=2;class=4")
modparam("dispatcher", "ds_probing_mode", 1)
modparam("dispatcher", "ds_ping_from", "sip:lb1 <at> m-lab-ca805-sig.kd-lab.de")
modparam("dispatcher", "dst_avp", "$avp(dsdst)")
modparam("dispatcher", "grp_avp", "$avp(dsgrp)")
modparam("dispatcher", "cnt_avp", "$avp(dscnt)")
modparam("dispatcher", "attrs_avp", "$avp(dsattrs)")
modparam("dispatcher", "dstid_avp", "$avp(dsdstid)")
modparam("dispatcher", "flags", 2)

But it is still give the following error:
0(1917) ERROR: dispatcher [dispatcher.c:624]: failover functions used, but AVPs paraamters required are NULL -- feature disabled

Does anybody know why?

Gary