diff --git a/modules/openapi-generator/src/main/resources/dart/libraries/dio/api.mustache b/modules/openapi-generator/src/main/resources/dart/libraries/dio/api.mustache
index bcb912d6c26..295c6a67d7b 100644
--- a/modules/openapi-generator/src/main/resources/dart/libraries/dio/api.mustache
+++ b/modules/openapi-generator/src/main/resources/dart/libraries/dio/api.mustache
@@ -44,10 +44,10 @@ class {{classname}} {
   {{/isDeprecated}}
   Future<Response<{{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}void{{/returnType}}>> {{nickname}}({ {{#allParams}}{{#isPathParam}}
     required {{{dataType}}} {{paramName}},{{/isPathParam}}{{#isQueryParam}}
-    {{#required}}required {{/required}}{{{dataType}}}{{^required}}?{{/required}} {{paramName}},{{/isQueryParam}}{{#isHeaderParam}}
-    {{#required}}required {{/required}}{{{dataType}}}{{^required}}?{{/required}} {{paramName}},{{/isHeaderParam}}{{#isBodyParam}}
-    {{#required}}required {{/required}}{{{dataType}}}{{^required}}?{{/required}} {{paramName}},{{/isBodyParam}}{{#isFormParam}}
-    {{#required}}required {{/required}}{{{dataType}}}{{^required}}?{{/required}} {{paramName}},{{/isFormParam}}{{/allParams}}
+    {{#required}}{{^isNullable}}required {{/isNullable}}{{/required}}{{{dataType}}}{{#required}}{{#isNullable}}?{{/isNullable}}{{/required}}{{^required}}?{{/required}} {{paramName}}{{^isContainer}}{{#defaultValue}} = {{{.}}}{{/defaultValue}}{{/isContainer}},{{/isQueryParam}}{{#isHeaderParam}}
+    {{#required}}{{^isNullable}}required {{/isNullable}}{{/required}}{{{dataType}}}{{#required}}{{#isNullable}}?{{/isNullable}}{{/required}}{{^required}}?{{/required}} {{paramName}}{{^isContainer}}{{#defaultValue}} = {{{.}}}{{/defaultValue}}{{/isContainer}},{{/isHeaderParam}}{{#isBodyParam}}
+    {{#required}}{{^isNullable}}required {{/isNullable}}{{/required}}{{{dataType}}}{{#required}}{{#isNullable}}?{{/isNullable}}{{/required}}{{^required}}?{{/required}} {{paramName}},{{/isBodyParam}}{{#isFormParam}}
+    {{#required}}{{^isNullable}}required {{/isNullable}}{{/required}}{{{dataType}}}{{#required}}{{#isNullable}}?{{/isNullable}}{{/required}}{{^required}}?{{/required}} {{paramName}},{{/isFormParam}}{{/allParams}}
     CancelToken? cancelToken,
     Map<String, dynamic>? headers,
     Map<String, dynamic>? extra,
diff --git a/modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/built_value/serialize.mustache b/modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/built_value/serialize.mustache
index d876fe2979b..54d8e99c757 100644
--- a/modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/built_value/serialize.mustache
+++ b/modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/built_value/serialize.mustache
@@ -16,7 +16,7 @@
       {{/hasFormParams}}
       {{#bodyParam}}
       {{#isPrimitiveType}}
-      _bodyData = {{paramName}}{{#isFile}}.finalize(){{/isFile}};
+      _bodyData = {{paramName}}{{#isFile}}{{#required}}{{#isNullable}}?{{/isNullable}}{{/required}}{{^required}}?{{/required}}.finalize(){{/isFile}};
       {{/isPrimitiveType}}
       {{^isPrimitiveType}}
       {{#isContainer}}
diff --git a/samples/openapi3/client/petstore/dart-dio-next/petstore_client_lib_fake/lib/src/api/fake_api.dart b/samples/openapi3/client/petstore/dart-dio-next/petstore_client_lib_fake/lib/src/api/fake_api.dart
index ad7e1f479fe..b65692f8859 100644
--- a/samples/openapi3/client/petstore/dart-dio-next/petstore_client_lib_fake/lib/src/api/fake_api.dart
+++ b/samples/openapi3/client/petstore/dart-dio-next/petstore_client_lib_fake/lib/src/api/fake_api.dart
@@ -647,7 +647,7 @@ class FakeApi {
   /// Returns a [Future]
   /// Throws [DioError] if API call or serialization fails
   Future<Response<void>> testBodyWithBinary({ 
-    required MultipartFile body,
+    MultipartFile? body,
     CancelToken? cancelToken,
     Map<String, dynamic>? headers,
     Map<String, dynamic>? extra,
@@ -672,7 +672,7 @@ class FakeApi {
     dynamic _bodyData;
 
     try {
-      _bodyData = body.finalize();
+      _bodyData = body?.finalize();
 
     } catch(error, stackTrace) {
       throw DioError(
@@ -1065,9 +1065,9 @@ class FakeApi {
   /// Throws [DioError] if API call or serialization fails
   Future<Response<void>> testEnumParameters({ 
     BuiltList<String>? enumHeaderStringArray,
-    String? enumHeaderString,
+    String? enumHeaderString = '-efg',
     BuiltList<String>? enumQueryStringArray,
-    String? enumQueryString,
+    String? enumQueryString = '-efg',
     int? enumQueryInteger,
     double? enumQueryDouble,
     BuiltList<String>? enumFormStringArray,