proxy.config.http.slow.log.threshold is unable to set from traffic_line -s

[root@cache164 trafficserver]# traffic_line -s proxy.config.http.slow.log.threshold -v 30
Layout configuration
          --prefix = '/usr'
     --exec_prefix = '/usr'
          --bindir = '/usr/bin'
         --sbindir = '/usr/sbin'
      --sysconfdir = '/etc/trafficserver'
         --datadir = '/usr/share/trafficserver'
      --includedir = '/usr/include/trafficserver'
          --libdir = '/usr/lib64/trafficserver'
      --libexecdir = '/usr/lib64/trafficserver/plugins'
   --localstatedir = '/var/trafficserver'
      --runtimedir = '/var/run/trafficserver'
          --logdir = '/var/log/trafficserver'
          --mandir = '/usr/share/man'
         --infodir = '/usr/share/info'
        --cachedir = '/var/cache/trafficserver'
traffic_line: Only configuration vars can be set