Error: no implicit conversion of LogStash::Util::Password into String

Version: logstash-input-cloudwatch-2.1.1/logstash 6.2.4
Operating System: Ubuntu 16.04.4 LTS

Config File (if you have sensitive info, please remove it):

cloudwatch {
access_key_id => "XXXXXXXXXX"
secret_access_key => "YYYYYYYYYYY"
interval => 300
namespace => "AWS/EC2"
metrics => [ "CPUCreditBalance", "CPUCreditUsage", "CPUSurplusCreditBalance", "CPUSurplusCreditsCharged", "CPUUtilization", "DiskReadBytes", "DiskReadOps", "DiskWriteBytes", "DiskWriteOps", "NetworkIn", "NetworkOut", "NetworkPacketsIn", "NetworkPacketsOut", "StatusCheckFailed", "StatusCheckFailed_Instance", "StatusCheckFailed_System" ]
region => "us-east-1"
filters => {
    "instance-state-code" => "running"
}
add_field => {
    "instance-state-code" => "running"
    "techstack" => "XYZ"
    "source" => "CloudWatch"
    "region" => "us-east-1"
}
}

Sample Data: NA
Steps to Reproduce: the same config file works fine in logstash-input-cloudwatch-2.0.3 with logstash 6.1.3. But I got this error if using logstash-input-cloudwatch-2.1.1/logstash 6.2.4

Error as follow Error log, config file could be found if scroll down

[2018-05-04T02:22:12,493][ERROR][logstash.pipeline        ] A plugin had an unrecoverable error. Will restart this plugin.
  Pipeline_id:main
  Plugin: <LogStash::Inputs::CloudWatch access_key_id=>"XXXXXXXXX", secret_access_key=><password>, interval=>300, namespace=>"AWS/EC2", metrics=>["CPUCreditBalance", "CPUCreditUsage", "CPUSurplusCreditBalance", "CPUSurplusCreditsCharged", "CPUUtilization", "DiskReadBytes", "DiskReadOps", "DiskWriteBytes", "DiskWriteOps", "NetworkIn", "NetworkOut", "NetworkPacketsIn", "NetworkPacketsOut", "StatusCheckFailed", "StatusCheckFailed_Instance", "StatusCheckFailed_System"], region=>"us-east-1", filters=>{"instance-state-code"=>"running"}, add_field=>{"instance-state-code"=>"running", "techstack"=>"ZZZZZZZZ", "source"=>"CloudWatch", "region"=>"us-east-1"}, id=>"XXXXXXXXXXXXX", enable_metric=>true, codec=><LogStash::Codecs::Plain id=>"XXXXXXX", enable_metric=>true, charset=>"UTF-8">, role_session_name=>"logstash", use_ssl=>true, statistics=>["SampleCount", "Average", "Minimum", "Maximum", "Sum"], period=>300, combined=>false>
  Error: no implicit conversion of LogStash::Util::Password into String
  Exception: TypeError
  Stack: org/jruby/RubyString.java:1144:in `+'
/usr/share/logstash/vendor/bundle/jruby/2.3.0/gems/aws-sdk-v1-1.67.0/lib/aws/core/signers/version_4.rb:93:in `derive_key'
/usr/share/logstash/vendor/bundle/jruby/2.3.0/gems/aws-sdk-v1-1.67.0/lib/aws/core/signers/version_4.rb:58:in `sign_request'
/usr/share/logstash/vendor/bundle/jruby/2.3.0/gems/aws-sdk-v1-1.67.0/lib/aws/core/client.rb:708:in `block in sign_request'
/usr/share/logstash/vendor/bundle/jruby/2.3.0/gems/aws-sdk-v1-1.67.0/lib/aws/core/client.rb:492:in `block in client_request'
/usr/share/logstash/vendor/bundle/jruby/2.3.0/gems/aws-sdk-v1-1.67.0/lib/aws/core/response.rb:175:in `build_request'
/usr/share/logstash/vendor/bundle/jruby/2.3.0/gems/aws-sdk-v1-1.67.0/lib/aws/core/response.rb:114:in `initialize'
/usr/share/logstash/vendor/bundle/jruby/2.3.0/gems/aws-sdk-v1-1.67.0/lib/aws/core/client.rb:203:in `new_response'
/usr/share/logstash/vendor/bundle/jruby/2.3.0/gems/aws-sdk-v1-1.67.0/lib/aws/core/client.rb:490:in `block in client_request'
/usr/share/logstash/vendor/bundle/jruby/2.3.0/gems/aws-sdk-v1-1.67.0/lib/aws/core/client.rb:391:in `log_client_request'
/usr/share/logstash/vendor/bundle/jruby/2.3.0/gems/aws-sdk-v1-1.67.0/lib/aws/core/client.rb:477:in `block in client_request'
/usr/share/logstash/vendor/bundle/jruby/2.3.0/gems/aws-sdk-v1-1.67.0/lib/aws/core/client.rb:373:in `return_or_raise'
/usr/share/logstash/vendor/bundle/jruby/2.3.0/gems/aws-sdk-v1-1.67.0/lib/aws/core/client.rb:476:in `client_request'
(eval):3:in `list_metrics'
/usr/share/logstash/vendor/bundle/jruby/2.3.0/gems/logstash-input-cloudwatch-2.1.1/lib/logstash/inputs/cloudwatch.rb:255:in `block in metrics_available'
org/jruby/RubyHash.java:711:in `default'
org/jruby/RubyHash.java:1100:in `[]'
/usr/share/logstash/vendor/bundle/jruby/2.3.0/gems/logstash-input-cloudwatch-2.1.1/lib/logstash/inputs/cloudwatch.rb:245:in `metrics_for'
/usr/share/logstash/vendor/bundle/jruby/2.3.0/gems/logstash-input-cloudwatch-2.1.1/lib/logstash/inputs/cloudwatch.rb:144:in `block in run'
/usr/share/logstash/vendor/bundle/jruby/2.3.0/gems/stud-0.0.23/lib/stud/interval.rb:20:in `interval'
/usr/share/logstash/vendor/bundle/jruby/2.3.0/gems/logstash-input-cloudwatch-2.1.1/lib/logstash/inputs/cloudwatch.rb:141:in `run'
/usr/share/logstash/logstash-core/lib/logstash/pipeline.rb:514:in `inputworker'
/usr/share/logstash/logstash-core/lib/logstash/pipeline.rb:507:in `block in start_input'

logstash-plugins / logstash-input-cloudwatch

Error: no implicit conversion of LogStash::Util::Password into String #30