Getting values of json from POST payload not working

[root@ve10:Active] config  b virtual bar list
virtual bar {
   snat automap
   pool foo
   destination 172.28.19.252:80
   ip protocol 6
   rules myrule
   profiles {
      http {}
      tcp {}
   }
}
[root@ve10:Active] config  b rule myrule list
rule myrule {
   when HTTP_REQUEST {
  if {[HTTP::method] eq "POST"}{
    if {[HTTP::header "Content-Length"] ne "" && [HTTP::header "Content-Length"] <= 1048576}{
      set content_length [HTTP::header "Content-Length"]
    } else {
        set content_length 1048576
    }
    if { $content_length > 0} {
      HTTP::collect $content_length
    }
  }
}
when HTTP_REQUEST_DATA {
  set a [URI::query "?&[HTTP::payload]" parA]
  set b [URI::query "?&[HTTP::payload]" parB]
  set c [URI::query "?&[HTTP::payload]" parC]
  set d [URI::query "?&[HTTP::payload]" parD]
  log local0. "a: $a b: $b c: $c d: $d"
}
}


 trace

[root@ve10:Active] config  ssldump -Aed -nni 0.0 port 80
New TCP connection 1: 172.28.20.17(50767) <-> 172.28.19.252(80)
1375483782.1026 (0.0012)  C>S
---------------------------------------------------------------
POST / HTTP/1.1
User-Agent: curl/7.15.5 (i686-redhat-linux-gnu) libcurl/7.15.5 OpenSSL/0.9.8b zlib/1.2.3 libidn/0.6.5
Host: 172.28.19.252
Accept: */*
Content-Length: 38
Content-Type: application/x-www-form-urlencoded

parA=one&parB=two&parC=three&parD=four---------------------------------------------------------------

 /var/log/ltm

[root@ve10:Active] config  cat /var/log/ltm
Aug  3 06:49:42 local/tmm info tmm[5139]: Rule myrule : a: one b: two c: three d: four