diff --git a/README.md b/README.md
index 6ce7d6d0..a524f9c3 100644
--- a/README.md
+++ b/README.md
@@ -82,17 +82,17 @@ bool_value = client.fetch_boolean_value(flag_key: 'boolean_flag', default_value:
 
 # a details method is also available for more information about the flag evaluation
 # see `ResolutionDetails` for more info
-bool_details = client.fetch_boolean_details(flag_key: 'boolean_flag', default_value: false) ==
+bool_details = client.fetch_boolean_details(flag_key: 'boolean_flag', default_value: false)
 
 # fetching string value feature flag
-string_value = client.fetch_string_value(flag_key: 'string_flag', default_value: false)
+string_value = client.fetch_string_value(flag_key: 'string_flag', default_value: 'default')
 
 # fetching number value feature flag
 float_value = client.fetch_number_value(flag_key: 'number_value', default_value: 1.0)
 integer_value = client.fetch_number_value(flag_key: 'number_value', default_value: 1)
 
 # get an object value
-object = client.fetch_object_value(flag_key: 'object_value', default_value: JSON.dump({ name: 'object'}))
+object = client.fetch_object_value(flag_key: 'object_value', default_value: { name: 'object'})
 ```
 
 ## 🌟 Features
diff --git a/lib/open_feature/sdk/client.rb b/lib/open_feature/sdk/client.rb
index f5fc98c3..7dee6928 100644
--- a/lib/open_feature/sdk/client.rb
+++ b/lib/open_feature/sdk/client.rb
@@ -44,6 +44,8 @@ def fetch_#{result_type}_#{suffix}(flag_key:, default_value:, evaluation_context
       private
 
       def fetch_details(type:, flag_key:, default_value:, evaluation_context: nil)
+        validate_default_value_type(type, default_value)
+
         built_context = EvaluationContextBuilder.new.call(api_context: OpenFeature::SDK.evaluation_context, client_context: self.evaluation_context, invocation_context: evaluation_context)
 
         resolution_details = @provider.send(:"fetch_#{type}_value", flag_key:, default_value:, evaluation_context: built_context)
@@ -56,6 +58,15 @@ def fetch_details(type:, flag_key:, default_value:, evaluation_context: nil)
 
         EvaluationDetails.new(flag_key:, resolution_details:)
       end
+
+      def validate_default_value_type(type, default_value)
+        expected_classes = TYPE_CLASS_MAP[type]
+        unless expected_classes.any? { |klass| default_value.is_a?(klass) }
+          expected_types = expected_classes.map(&:name).join(" or ")
+          actual_type = default_value.class.name
+          raise ArgumentError, "Default value for #{type} must be #{expected_types}, got #{actual_type}"
+        end
+      end
     end
   end
 end
diff --git a/spec/open_feature/sdk/client_spec.rb b/spec/open_feature/sdk/client_spec.rb
index b8c404fc..74c4722d 100644
--- a/spec/open_feature/sdk/client_spec.rb
+++ b/spec/open_feature/sdk/client_spec.rb
@@ -353,5 +353,83 @@
         pending
       end
     end
+
+    context "Default Value Type Validation" do
+      let(:flag_key) { "test-flag" }
+
+      context "Valid default value types" do
+        it "accepts boolean values for fetch_boolean_value" do
+          expect { client.fetch_boolean_value(flag_key:, default_value: true) }.not_to raise_error
+          expect { client.fetch_boolean_value(flag_key:, default_value: false) }.not_to raise_error
+        end
+
+        it "accepts string values for fetch_string_value" do
+          expect { client.fetch_string_value(flag_key:, default_value: "test") }.not_to raise_error
+          expect { client.fetch_string_value(flag_key:, default_value: "") }.not_to raise_error
+        end
+
+        it "accepts numeric values for fetch_number_value" do
+          expect { client.fetch_number_value(flag_key:, default_value: 42) }.not_to raise_error
+          expect { client.fetch_number_value(flag_key:, default_value: 3.14) }.not_to raise_error
+        end
+
+        it "accepts integer values for fetch_integer_value" do
+          expect { client.fetch_integer_value(flag_key:, default_value: 42) }.not_to raise_error
+        end
+
+        it "accepts float values for fetch_float_value" do
+          expect { client.fetch_float_value(flag_key:, default_value: 3.14) }.not_to raise_error
+        end
+
+        it "accepts array and hash values for fetch_object_value" do
+          expect { client.fetch_object_value(flag_key:, default_value: {}) }.not_to raise_error
+          expect { client.fetch_object_value(flag_key:, default_value: []) }.not_to raise_error
+        end
+      end
+
+      context "Invalid default value types" do
+        it "raises ArgumentError for invalid boolean default values" do
+          expect { client.fetch_boolean_value(flag_key:, default_value: "not boolean") }
+            .to raise_error(ArgumentError, /Default value for boolean must be TrueClass or FalseClass, got String/)
+          expect { client.fetch_boolean_value(flag_key:, default_value: 1) }
+            .to raise_error(ArgumentError, /Default value for boolean must be TrueClass or FalseClass, got Integer/)
+        end
+
+        it "raises ArgumentError for invalid string default values" do
+          expect { client.fetch_string_value(flag_key:, default_value: true) }
+            .to raise_error(ArgumentError, /Default value for string must be String, got TrueClass/)
+          expect { client.fetch_string_value(flag_key:, default_value: 42) }
+            .to raise_error(ArgumentError, /Default value for string must be String, got Integer/)
+        end
+
+        it "raises ArgumentError for invalid number default values" do
+          expect { client.fetch_number_value(flag_key:, default_value: "42") }
+            .to raise_error(ArgumentError, /Default value for number must be Numeric, got String/)
+          expect { client.fetch_number_value(flag_key:, default_value: true) }
+            .to raise_error(ArgumentError, /Default value for number must be Numeric, got TrueClass/)
+        end
+
+        it "raises ArgumentError for invalid integer default values" do
+          expect { client.fetch_integer_value(flag_key:, default_value: 3.14) }
+            .to raise_error(ArgumentError, /Default value for integer must be Integer, got Float/)
+          expect { client.fetch_integer_value(flag_key:, default_value: "42") }
+            .to raise_error(ArgumentError, /Default value for integer must be Integer, got String/)
+        end
+
+        it "raises ArgumentError for invalid float default values" do
+          expect { client.fetch_float_value(flag_key:, default_value: 42) }
+            .to raise_error(ArgumentError, /Default value for float must be Float, got Integer/)
+          expect { client.fetch_float_value(flag_key:, default_value: "3.14") }
+            .to raise_error(ArgumentError, /Default value for float must be Float, got String/)
+        end
+
+        it "raises ArgumentError for invalid object default values" do
+          expect { client.fetch_object_value(flag_key:, default_value: "not object") }
+            .to raise_error(ArgumentError, /Default value for object must be Array or Hash, got String/)
+          expect { client.fetch_object_value(flag_key:, default_value: 42) }
+            .to raise_error(ArgumentError, /Default value for object must be Array or Hash, got Integer/)
+        end
+      end
+    end
   end
 end