logstash base64 decode with ruby filter

ruby_filter_base64_decode_example.sh

$ cat base64_decode.conf
input {
  stdin { }
}

filter {
  grok {
    match => ["message", "%{WORD:prefix} %{WORD:b64} %{WORD:suffix}"]
  }

  ruby {
    init => "require 'base64'"
    code => "event['b64_decoded'] = Base64.decode64(event['b64']) if event.include?('b64')"
  }
}

output {
  stdout {
    codec => rubydebug
  }
}


# messages:
#   p bWlkZGxl s
#   p middle s
#   p  s
$ echo "p bWlkZGxl s\np middle s\np  s" | ./bin/logstash agent -f base64_decode.conf
[deprecated] I18n.enforce_available_locales will default to true in the future. If you really want to skip validation of your locale you can set I18n.enforce_available_locales = false to avoid this message.
Using milestone 1 filter plugin 'ruby'. This plugin should work, but would benefit from use by folks like you. Please let us know if you find bugs or have suggestions on how to improve this plugin.  For more information on plugin milestones, see http://logstash.net/docs/1.2.3.dev/plugin-milestones {:level=>:warn}
{
        "message" => "p bWlkZGxl s",
     "@timestamp" => "2013-12-07T04:09:16.662Z",
       "@version" => "1",
           "host" => "t61",
         "prefix" => "p",
            "b64" => "bWlkZGxl",
         "suffix" => "s",
    "b64_decoded" => "middle"
}
{
        "message" => "p middle s",
     "@timestamp" => "2013-12-07T04:09:16.663Z",
       "@version" => "1",
           "host" => "t61",
         "prefix" => "p",
            "b64" => "middle",
         "suffix" => "s",
    "b64_decoded" => "\x9A']"
}
{
       "message" => "p  s",
    "@timestamp" => "2013-12-07T04:09:16.663Z",
      "@version" => "1",
          "host" => "t61",
          "tags" => [
        [0] "_grokparsefailure"
    ]
}

Hi,
I am trying with below logstash conf file to decode base64 logs to send decoded data to elasticsearch but its throwing below error -
[2019-07-17T19:01:36,791][ERROR][logstash.filters.ruby ] Ruby exception occurred: undefined method `unpack1' for nil:NilClass

Logstash.conf file

input {
s3 {
access_key_id => "xxxxxxx"
bucket => "kinesis-datadog"
region => "us-east-1"
secret_access_key => "xxxxxxx8"
prefix => "2019/07/05"
type => "S3"
}
}

filter {
grok {
match => {"message" => "%{WORD:prefix} %{WORD:b64} %{WORD:suffix}"}
}
ruby {
init => "require 'base64'"
code => "event['b64_decoded'] = Base64.decode64(event['b64']) if event.include?('b64')"
#code => "event.set('[decodedBase64]', Base64.decode64(event.get('[base64_field]')))"
}
}

#output plugin, which has the ElasticSearch domain information

output {

elasticsearch {
hosts => ["http://localhost:9200"]
index => ".kibana-%{+YYYY.MM.dd}"
codec => rubydebug
#user => "elastic"
#password => "elastic"
}
}