TypeError Occurs When Map Data and Headers are Set Together

[cizmarf]

Description

If Map response data and headers are set together it throws the exception
Error: TypeError: Instance of 'IdentityMap<String, String>': type 'IdentityMap<String, String>' is not a subtype of type 'String'.

Steps to reproduce

1. Define mocked route

  dioAdapter.onGet(
    'https://example.com/user/john',
    (server) => server.reply(
      200,
      {
        'name': 'John',
        'surname': 'Doe',
      },
      headers: {
        'set-cookie': ['a', 'b']
      },
      delay: const Duration(seconds: 1),
    ),
  );

2. Call the endpoint

Response response = await dio.get(
      'https://example.com/user/john',
    );

3. Catch the exception

Expected behavior

print(response.data); should print {name: John, surname: Doe}.

Screenshots

No screenshots

System details

flutter doctor

Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 3.22.0, on macOS 14.3 23D5033f darwin-arm64 (Rosetta), locale en-US)
[✓] Android toolchain - develop for Android devices (Android SDK version 34.0.0)
[✓] Xcode - develop for iOS and macOS (Xcode 15.4)
[✓] Chrome - develop for the web
[✓] Android Studio (version 2023.2)
[✓] VS Code (version 1.90.0)
[✓] Connected device (4 available)
    ! Error: Browsing on the local area network for Filip‘s iPad. Ensure the device is unlocked and attached with a cable or associated with the
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[✓] Network resources

• No issues found!

pubspec.yaml

environment:
  sdk: '>=3.4.0 <4.0.0'

dependencies:
  dio: ^5.4.3+1
  flutter:
    sdk: flutter
  http_mock_adapter: ^0.6.1

Additional context

Here is the minimal not working app.
https://github.com/cizmarf/HttpMockAdapterBugReport/blob/main/lib/main.dart

Its output is:

Fetching Foo, Map body without headers works
{name: Foo, surname: Bar}
Fetching Foo finished

Fetching Jane, String body with headers works
{
        'name': 'Jane',
        'surname': 'Doe',
      }
Fetching Jane finished

Fetching John, Map body with headers DOES NOT work
ERROR HAPPENS HERE
DioException [unknown]: null
Error: TypeError: Instance of 'IdentityMap<String, String>': type 'IdentityMap<String, String>' is not a subtype of type 'String'
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 296:3  throw_
packages/dio/src/dio_mixin.dart 509:7                                        fetch
dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 60:31           <fn>
dart-sdk/lib/async/zone.dart 1666:54                                         runBinary
dart-sdk/lib/async/future_impl.dart 178:22                                   handleError
dart-sdk/lib/async/future_impl.dart 859:46                                   handleError
dart-sdk/lib/async/future_impl.dart 880:13                                   _propagateToListeners
dart-sdk/lib/async/future_impl.dart 660:5                                    [_completeError]
dart-sdk/lib/async/future_impl.dart 746:7                                    callback
dart-sdk/lib/async/schedule_microtask.dart 40:11                             _microtaskLoop
dart-sdk/lib/async/schedule_microtask.dart 49:5                              _startMicrotaskLoop
dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 181:7           <fn>

Fetching John finished