diff --git a/lib/aws_lambda_ric/aws_lambda_marshaller.rb b/lib/aws_lambda_ric/aws_lambda_marshaller.rb
index 7d712b0..877229b 100644
--- a/lib/aws_lambda_ric/aws_lambda_marshaller.rb
+++ b/lib/aws_lambda_ric/aws_lambda_marshaller.rb
@@ -2,6 +2,8 @@
 
 # frozen_string_literal: true
 
+require 'stringio'
+
 module AwsLambda
   class Marshaller
     class << self
diff --git a/test/unit/resources/runtime_handlers/core.rb b/test/unit/resources/runtime_handlers/core.rb
index b21268b..fe306a6 100644
--- a/test/unit/resources/runtime_handlers/core.rb
+++ b/test/unit/resources/runtime_handlers/core.rb
@@ -1,5 +1,7 @@
 # Copyright 2018 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 
+require 'stringio'
+
 def ping(event:, context:)
   resp = {}
   if event.nil?