NoMethodError: undefined method `start_with?' for nil:NilClass

Hi,
I am trying to send logs to zabbix server using logstash zabbix output.

Sample log file contents

2015-11-11 10:27:40.8798 FATAL Some Service Providers are missing data required by FAS: TestServiceProvider-7
2015-11-11 10:29:40.8798 FATAL Some Service Providers are missing data required by FAS: TestServiceProvider-7

logstash.conf file as below

input {

file {
  type => "FASLoader_alert"
  path => "/opt/logstash/bin/fasAlerts/FASLoader_Alert.log"
}

}

filter {
   grok{

     match => [ "@message", "(error|ERROR|CRITICAL|FATAL)" ]
     add_tag => [ "fas-alerts" ]

   # add_field => { "zabbix_host" => "ELK" "zkey" => "hello"}
   }
mutate {
  add_field => { "[@metadata][zabbix_host]" => "log[/opt/logstash/bin/fasAlerts/FASLoader_Alert.log,FATAL]" }
}
   }

output {
stdout { codec => rubydebug }
  zabbix {
        zabbix_host => "[@metadata][zabbix_host]"
        

 }
}

When I start logstash, then it shows following error.

Logstash startup completed
{
       "message" => "2015-09-03 12:00:40.8798 FATAL Some Service Providers are missing data required by FAS: TestServiceProvider-1",
      "@version" => "1",
    "@timestamp" => "2015-11-20T09:34:09.794Z",
          "host" => "host-namei",
          "path" => "/opt/logstash/bin/fasAlerts/FASLoader_Alert.log",
          "type" => "FASLoader_alert",
          "tags" => [
        [0] "_grokparsefailure"
    ]
}
NoMethodError: undefined method `start_with?' for nil:NilClass
               [] at /opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-1.5.3-java/lib/logstash/event.rb:126
      field_check at /opt/logstash/vendor/local_gems/9054c277/logstash-output-zabbix-1.0.0/lib/logstash/outputs/zabbix.rb:104
         kv_check at /opt/logstash/vendor/local_gems/9054c277/logstash-output-zabbix-1.0.0/lib/logstash/outputs/zabbix.rb:116
             each at org/jruby/RubyArray.java:1613
         kv_check at /opt/logstash/vendor/local_gems/9054c277/logstash-output-zabbix-1.0.0/lib/logstash/outputs/zabbix.rb:115
  validate_fields at /opt/logstash/vendor/local_gems/9054c277/logstash-output-zabbix-1.0.0/lib/logstash/outputs/zabbix.rb:125
             step at org/jruby/RubyRange.java:578
  validate_fields at /opt/logstash/vendor/local_gems/9054c277/logstash-output-zabbix-1.0.0/lib/logstash/outputs/zabbix.rb:124
   format_request at /opt/logstash/vendor/local_gems/9054c277/logstash-output-zabbix-1.0.0/lib/logstash/outputs/zabbix.rb:138
         tcp_send at /opt/logstash/vendor/local_gems/9054c277/logstash-output-zabbix-1.0.0/lib/logstash/outputs/zabbix.rb:201
             open at org/jruby/RubyIO.java:1183
         tcp_send at /opt/logstash/vendor/local_gems/9054c277/logstash-output-zabbix-1.0.0/lib/logstash/outputs/zabbix.rb:200
   send_to_zabbix at /opt/logstash/vendor/local_gems/9054c277/logstash-output-zabbix-1.0.0/lib/logstash/outputs/zabbix.rb:223
          timeout at org/jruby/ext/timeout/Timeout.java:126
   send_to_zabbix at /opt/logstash/vendor/local_gems/9054c277/logstash-output-zabbix-1.0.0/lib/logstash/outputs/zabbix.rb:222
          receive at /opt/logstash/vendor/local_gems/9054c277/logstash-output-zabbix-1.0.0/lib/logstash/outputs/zabbix.rb:239
           handle at /opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-1.5.3-java/lib/logstash/outputs/base.rb:88
      output_func at (eval):78
     outputworker at /opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-1.5.3-java/lib/logstash/pipeline.rb:244
    start_outputs at /opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-core-1.5.3-java/lib/logstash/pipeline.rb:166

Can anybody tell whats wrong in my configuration?

Zabbix monitoring is running on 127.0.0.1.
I have created 2 hosts form zabbix frontend as below

Hi all,
any guess what might be the issue?
Please help.

thanks
Sunil.