diff --git a/packages/openapi_code_builder/example/pubspec.lock b/packages/openapi_code_builder/example/pubspec.lock
index 8bb9c23..7bb9b1e 100644
--- a/packages/openapi_code_builder/example/pubspec.lock
+++ b/packages/openapi_code_builder/example/pubspec.lock
@@ -366,14 +366,14 @@ packages:
       path: "../../openapi_base"
       relative: true
     source: path
-    version: "1.1.0+3"
+    version: "1.3.1"
   openapi_code_builder:
     dependency: "direct dev"
     description:
       path: ".."
       relative: true
     source: path
-    version: "1.2.0"
+    version: "1.3.0"
   package_config:
     dependency: transitive
     description:
@@ -470,6 +470,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.10.0"
+  sprintf:
+    dependency: transitive
+    description:
+      name: sprintf
+      sha256: "1fc9ffe69d4df602376b52949af107d8f5703b77cda567c4d7d86a0693120f23"
+      url: "https://pub.dev"
+    source: hosted
+    version: "7.0.0"
   stack_trace:
     dependency: transitive
     description:
@@ -546,10 +554,10 @@ packages:
     dependency: transitive
     description:
       name: uuid
-      sha256: "648e103079f7c64a36dc7d39369cabb358d377078a051d6ae2ad3aa539519313"
+      sha256: "814e9e88f21a176ae1359149021870e87f7cddaf633ab678a5d2b0bff7fd1ba8"
       url: "https://pub.dev"
     source: hosted
-    version: "3.0.7"
+    version: "4.4.0"
   watcher:
     dependency: transitive
     description:
diff --git a/packages/openapi_code_builder/lib/src/openapi_code_builder.dart b/packages/openapi_code_builder/lib/src/openapi_code_builder.dart
index caf1847..833dbb1 100644
--- a/packages/openapi_code_builder/lib/src/openapi_code_builder.dart
+++ b/packages/openapi_code_builder/lib/src/openapi_code_builder.dart
@@ -72,6 +72,7 @@ class OpenApiLibraryGenerator {
       refer('OpenApiClientBase', 'package:openapi_base/openapi_base.dart');
   final _hasSuccessResponse =
       refer('HasSuccessResponse', 'package:openapi_base/openapi_base.dart');
+
 //  final _openApiHttpHeaders =
 //      refer('OpenApiHttpHeaders', 'package:openapi_base/openapi_base.dart');
   final _openApiClientRequestBodyJson = refer(
@@ -1277,9 +1278,11 @@ class OpenApiLibraryGenerator {
           ..name = '_additionalProperties'
           ..type = _referType(
             'Map',
-            generics: [_typeString, refer('Object')],
+            generics: [_typeString, _referType('Object', isNullable: true)],
           )
-          ..assignment = literalMap({}, _typeString, refer('Object')).code
+          ..assignment = literalMap(
+                  {}, _typeString, _referType('Object', isNullable: true))
+              .code
           ..modifier = FieldModifier.final$));
         cb.methods.add(
           Method((mb) => mb
@@ -1300,7 +1303,7 @@ class OpenApiLibraryGenerator {
         cb.methods.add(
           Method((mb) => mb
             ..name = 'operator[]'
-            ..returns = refer('Object')
+            ..returns = _referType('Object', isNullable: true)
             ..requiredParameters.add(Parameter((pb) => pb
               ..name = 'key'
               ..type = _typeString))
@@ -1584,11 +1587,16 @@ class OpenApiCodeBuilder extends Builder {
       };
 }
 
-TypeReference _referType(String name,
-        {String? url, List<Reference>? generics}) =>
+TypeReference _referType(
+  String name, {
+  String? url,
+  List<Reference>? generics,
+  bool? isNullable,
+}) =>
     TypeReference((trb) => trb
       ..symbol = name
       ..url = url
+      ..isNullable = isNullable
       ..types.addAll(generics!));
 
 extension on Reference {
@@ -1650,6 +1658,7 @@ extension DynamicExt<T> on dynamic {
 
 extension ObjectExt<T> on T {
   T? takeIf(bool Function(T that) predicate) => predicate(this) ? this : null;
+
   R let<R>(R Function(T that) op) => op(this);
 }