diff --git a/modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/built_value/serializers.mustache b/modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/built_value/serializers.mustache
index d59a148bae94..58ec9562052e 100644
--- a/modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/built_value/serializers.mustache
+++ b/modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/built_value/serializers.mustache
@@ -29,7 +29,7 @@ Serializers serializers = (_$serializers.toBuilder(){{#builtValueSerializers}}
 {{/isArray}}
 {{#isMap}}
         const FullType(BuiltMap, [FullType(String), FullType{{#isNullable}}.nullable{{/isNullable}}({{dataType}})]),
-        () => MapBuilder<String, {{dataType}}>(),
+        () => MapBuilder<String, {{dataType}}{{#isNullable}}?{{/isNullable}}>(),
 {{/isMap}}
       ){{/builtValueSerializers}}
       {{#models}}{{#model}}{{#vendorExtensions.x-is-parent}}..add({{classname}}.serializer)
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/src/serializers.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/src/serializers.dart
index 9ebe575fcf79..f55ca699ac78 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/src/serializers.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/src/serializers.dart
@@ -154,7 +154,7 @@ Serializers serializers = (_$serializers.toBuilder()
       )
       ..addBuilderFactory(
         const FullType(BuiltMap, [FullType(String), FullType.nullable(JsonObject)]),
-        () => MapBuilder<String, JsonObject>(),
+        () => MapBuilder<String, JsonObject?>(),
       )
       ..addBuilderFactory(
         const FullType(BuiltMap, [FullType(String), FullType(int)]),