diff --git a/CHANGELOG.md b/CHANGELOG.md
index 70dcce6..89b3630 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## 0.9.1
+
+### Bug fixes
+
+- Prevent mutable default options from being shared across instances.
+
 ## 0.9.0
 
 ### New features
@@ -21,6 +27,7 @@
 ## 0.8.1
 
 ## Maintenance
+
 - Loosen the ActiveSupport dependency version to prepare for Rails 7.
 
 ## 0.8.0
diff --git a/Gemfile.lock b/Gemfile.lock
index 0d3cb90..e119590 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -1,24 +1,23 @@
 PATH
   remote: .
   specs:
-    stimpack (0.8.3)
+    stimpack (0.9.1)
       activesupport (>= 6.1)
 
 GEM
   remote: https://rubygems.org/
   specs:
-    activesupport (6.1.4.1)
+    activesupport (7.0.2.2)
       concurrent-ruby (~> 1.0, >= 1.0.2)
       i18n (>= 1.6, < 2)
       minitest (>= 5.1)
       tzinfo (~> 2.0)
-      zeitwerk (~> 2.3)
     ast (2.4.2)
     concurrent-ruby (1.1.9)
     diff-lcs (1.4.4)
-    i18n (1.8.10)
+    i18n (1.10.0)
       concurrent-ruby (~> 1.0)
-    minitest (5.14.4)
+    minitest (5.15.0)
     parallel (1.20.1)
     parser (3.0.0.0)
       ast (~> 2.4.1)
@@ -54,7 +53,6 @@ GEM
     tzinfo (2.0.4)
       concurrent-ruby (~> 1.0)
     unicode-display_width (2.0.0)
-    zeitwerk (2.5.1)
 
 PLATFORMS
   x86_64-darwin-19
diff --git a/lib/stimpack/options_declaration/option.rb b/lib/stimpack/options_declaration/option.rb
index 7a07349..8f1410b 100644
--- a/lib/stimpack/options_declaration/option.rb
+++ b/lib/stimpack/options_declaration/option.rb
@@ -23,7 +23,7 @@ def initialize(name, required:, default:, transform:)
       def default_value
         return nil unless default?
 
-        default.respond_to?(:call) ? default.() : default
+        default.respond_to?(:call) ? default.() : default.dup
       end
 
       def transformed_value(value)
diff --git a/lib/stimpack/version.rb b/lib/stimpack/version.rb
index 3301ca1..efdac0f 100644
--- a/lib/stimpack/version.rb
+++ b/lib/stimpack/version.rb
@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 module Stimpack
-  VERSION = "0.9.0"
+  VERSION = "0.9.1"
 end
diff --git a/spec/stimpack/event_source_spec.rb b/spec/stimpack/event_source_spec.rb
index 9c48202..c0c6057 100644
--- a/spec/stimpack/event_source_spec.rb
+++ b/spec/stimpack/event_source_spec.rb
@@ -37,12 +37,12 @@ def self.to_s
   end
 
   describe ".on" do
-    context "with single event" do 
+    context "with single event" do
       it { expect { service.on(:foo) {} }.to change { klass.event_listeners["Foo.foo"].size }.by(1) }
     end
 
     context "with multiple events" do
-      it { expect { service.on(:foo, :bar, :qux) {} }.to change { klass.event_listeners["Foo.foo"].size }.by(1).and change { klass.event_listeners["Foo.bar"].size }.by(1).and change { klass.event_listeners["Foo.qux"].size }.by(1) }
+      it { expect { service.on(:foo, :bar, :qux) {} }.to change { klass.event_listeners["Foo.foo"].size }.by(1).and change { klass.event_listeners["Foo.bar"].size }.by(1).and change { klass.event_listeners["Foo.qux"].size }.by(1) } # rubocop:disable Layout/LineLength
     end
   end
 
diff --git a/spec/stimpack/options_declaration_spec.rb b/spec/stimpack/options_declaration_spec.rb
index 4e02269..b99c4c1 100644
--- a/spec/stimpack/options_declaration_spec.rb
+++ b/spec/stimpack/options_declaration_spec.rb
@@ -19,24 +19,29 @@
       option :grault, default: "bar", transform: ->(value) { value.upcase }
       option :garply, default: "baz", transform: :upcase
       option :waldo, required: false, transform: :to_sym
+      option :plugh, default: []
+
+      def call
+        plugh << "Hello"
+      end
     end
   end
 
   describe ".option" do
-    it { expect(service.options_configuration.size).to eq(10) }
+    it { expect(service.options_configuration.size).to eq(11) }
     it { expect(service.options_configuration.values).to all(be_a(described_class::Option)) }
 
     describe "private_reader (option)" do
       let(:public_instance_methods) { service.public_instance_methods(false) }
       let(:private_instance_methods) { service.private_instance_methods(false) }
 
-      it { expect(public_instance_methods).to contain_exactly(:baz) }
-      it { expect(private_instance_methods).to contain_exactly(:foo, :bar, :qux, :quux, :quuz, :corge, :grault, :garply, :waldo) } # rubocop:disable Layout/LineLength
+      it { expect(public_instance_methods).to include(:baz) }
+      it { expect(private_instance_methods).to include(:foo, :bar, :qux, :quux, :quuz, :corge, :grault, :garply, :waldo, :plugh) } # rubocop:disable Layout/LineLength
     end
   end
 
   describe ".options" do
-    it { expect(service.options).to contain_exactly(:foo, :bar, :baz, :qux, :quux, :quuz, :corge, :grault, :garply, :waldo) } # rubocop:disable Layout/LineLength
+    it { expect(service.options).to contain_exactly(:foo, :bar, :baz, :qux, :quux, :quuz, :corge, :grault, :garply, :waldo, :plugh) } # rubocop:disable Layout/LineLength
   end
 
   describe ".required_options" do
@@ -44,11 +49,11 @@
   end
 
   describe ".optional_options" do
-    it { expect(service.optional_options).to contain_exactly(:bar, :qux, :quux, :quuz, :grault, :garply, :waldo) }
+    it { expect(service.optional_options).to contain_exactly(:bar, :qux, :quux, :quuz, :grault, :garply, :waldo, :plugh) } # rubocop:disable Layout/LineLength
   end
 
   describe ".default_options" do
-    it { expect(service.default_options).to contain_exactly(:qux, :quux, :quuz, :grault, :garply) }
+    it { expect(service.default_options).to contain_exactly(:qux, :quux, :quuz, :grault, :garply, :plugh) }
   end
 
   describe "#initialize" do
@@ -100,6 +105,12 @@
         it { expect(instance.send(:quuz)).to eq(nil) }
       end
 
+      context "when default option is a mutable object" do
+        before { klass.new(foo: "1", baz: "2", corge: "3").() }
+
+        it { expect(instance.send(:plugh)).to eq([]) }
+      end
+
       context "when transform is applied to user input" do
         it { expect(instance.send(:corge)).to eq("FOO") }
       end