diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 85587aa..6b55dca 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -9,10 +9,9 @@ jobs:
       fail-fast: false
       matrix:
         os: [ ubuntu-latest, macos-latest ]
-        # TODO: Wait for supporting Ruby 3.0 in simplecov-cobertura.
-        # See https://github.com/dashingrocket/simplecov-cobertura/pull/16
-        # ruby: [ 2.5, 2.6, 2.7, 3.0 ]
-        ruby: [ 2.5, 2.6, 2.7 ]
+        ruby: [ '3.0', '3.1', '3.2', '3.3' ]
+        exclude:
+         - { os: macos-latest, ruby: '2.5' }
     runs-on: ${{ matrix.os }}
 
     steps:
diff --git a/lib/pb/serializer.rb b/lib/pb/serializer.rb
index f8c97e7..5493f26 100644
--- a/lib/pb/serializer.rb
+++ b/lib/pb/serializer.rb
@@ -22,11 +22,11 @@ class Configuration
       #   @return [Logger]
       attr_accessor :logger
       # @!attribute [r] missing_field_behavior
-      #   @return [:raise, :warn, :ignore] default: `:raise`
+      #   @return [:raise, :warn, :ignore] default: `:ignore`
       attr_reader :missing_field_behavior
 
       def initialize
-        self.missing_field_behavior = :raise
+        self.missing_field_behavior = :ignore
         self.logger = Logger.new(STDOUT)
       end
 
@@ -70,7 +70,7 @@ def build_default_mask(descriptor)
             case fd.type
             when :message
               case fd.submsg_name
-              when "google.protobuf.Timestamp", 
+              when "google.protobuf.Timestamp",
                 "google.protobuf.StringValue",
                 "google.protobuf.Int32Value" ,
                 "google.protobuf.Int64Value" ,
diff --git a/pb-serializer.gemspec b/pb-serializer.gemspec
index 5054d61..7e6dfba 100644
--- a/pb-serializer.gemspec
+++ b/pb-serializer.gemspec
@@ -38,6 +38,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "rspec", "~> 3.0"
   spec.add_development_dependency "rubocop", "0.67.2" # for onkcop
   spec.add_development_dependency "sqlite3", "~> 1.4"
-  spec.add_development_dependency "simplecov", "~> 0.18.5"
-  spec.add_development_dependency "simplecov-cobertura", "~> 1.3"
+  spec.add_development_dependency "simplecov", "~> 0.19"
+  spec.add_development_dependency "simplecov-cobertura", "~> 3.1"
+  spec.add_development_dependency "concurrent-ruby", "1.3.4"
 end
diff --git a/spec/examples/errors_spec.rb b/spec/examples/errors_spec.rb
index 5af53d2..7c4f5c9 100644
--- a/spec/examples/errors_spec.rb
+++ b/spec/examples/errors_spec.rb
@@ -39,7 +39,7 @@
       allow(Pb::Serializer).to receive(:configuration).and_return(config)
 
       Pb::Serializer.configure do |c|
-        c.missing_field_behavior = missing_field_behavior
+        c.missing_field_behavior = missing_field_behavior if missing_field_behavior
         c.logger = Logger.new(log_buffer)
       end
     end
@@ -72,6 +72,29 @@
       it { expect(log_buffer.string).to be_empty }
       it { is_expected.to be_a TestFixture::Simple::Message }
     end
+
+    context 'when missing_field_behavior is not configured (default behavior)' do
+      before do
+        # Reset configuration to test default behavior
+        Pb::Serializer.instance_variable_set(:@configuraiton, nil)
+        allow(Pb::Serializer).to receive(:configuration).and_call_original
+
+        # Don't configure missing_field_behavior to test default
+        Pb::Serializer.configure do |c|
+          c.logger = Logger.new(log_buffer)
+        end
+      end
+
+      # Override the parent's before block to avoid setting missing_field_behavior
+      let(:missing_field_behavior) { nil }
+
+      it 'uses default value :ignore' do
+        expect(Pb::Serializer.configuration.missing_field_behavior).to eq(:ignore)
+      end
+
+      it { expect(log_buffer.string).to be_empty }
+      it { is_expected.to be_a TestFixture::Simple::Message }
+    end
   end
 
   it 'raises when set invalid param to missing_field_behavior config' do