Logstash crashing "can't convert Fixnum into String"

[joseph4325 (joseph)]

I put in a filter recently which seemed to be working fine at first, but it would crash logstash every couple of hours with the following message:

{
    "level": "FATAL",
    "logEvent": {
        "error": {
            "metaClass": {
                "metaClass": {
                    "metaClass": {
                        "backtrace": [
                            "org/jruby/RubyString.java:4462:in `include?'",
                            "(eval):274760:in `initialize'",
                            "org/jruby/RubyArray.java:1613:in `each'",
                            "(eval):274758:in `initialize'",
                            "org/jruby/RubyProc.java:281:in `call'",
                            "(eval):21975:in `filter_func'",
                            "/usr/share/logstash/logstash-core/lib/logstash/pipeline.rb:398:in `filter_batch'",
                            "/usr/share/logstash/logstash-core/lib/logstash/pipeline.rb:379:in `worker_loop'",
                            "/usr/share/logstash/logstash-core/lib/logstash/pipeline.rb:342:in `start_workers'"
                        ],
                        "error": "can't convert Fixnum into String"
                    }
                }
            }
        },
        "message": "An unexpected error occurred!"
    },
    "loggerName": "logstash.runner",
    "thread": "LogStash::Runner",
    "timeMillis": 1542319501747
}

The filter that errors looks like this:

filter {
  if [type] == "elasticsearch-index-name" {
    grok {
      patterns_dir => '/etc/logstash/patterns/custom-pattern/'
      match => [ "message", "%{CUSTOM_GROK}" ]
    }
    if !("_grokparsefailure" in [tags]) {
      date {
        match => ["timestamp", "MMM dd HH:mm:ss", "MMM d HH:mm:ss" ]
      }
      ruby {
        code => "event.set('elk_lag', event.get('elk_recv_timestamp')-event.get('@timestamp'))"
      }
    json {
          source => "data"
        }
    }
    mutate {
      add_field => { "rotation_period" => "{{ rotation_period }}" }
      add_field => { "tgt_index" => "elasticsearch-index-name" }
    }
  }

It stops breaking if i revert the filter to this:

filter {
  if [type] == "elasticsearch-index-name" {
    mutate {
      add_field => { "rotation_period" => "{{ rotation_period }}" }
      add_field => { "tgt_index" => "elasticsearch-index-name" }
    }
  }
}

If i feed a log file that would break it through the input plugin everything works fine. The only problem is when i start logstash in production with all the other filters things break.