Getitng '_InternalLinkedHashMap

I'm trying to fetch categories data from API which works as expected. When I am trying to test it and I am running into the following error

type '_InternalLinkedHashMap' is not a subtype of type 'List'

package:test_flutter/service.dart 32:12 CategoryService.getCats

service.dart

class Service {
  String apiUrl = "...";


  http.Client _client;

  service() {
   _client = IOClient();
  }

 Service.customClient(this._client);

  Future<List<Categories>> getCats() async {
    final response = await _client.get(
      apiUrl + "/categories",
      headers: {
        'Content-Type': 'application/json',
        'Authorization': 'Bearer $token',
      },
    );

    if(response.statusCode == 200 || response.statusCode == 201) {
      print('Response status: ${response.statusCode}');
      print('Response body: ${response.body}');
      List jsonList = json.decode(response.body);

      List<Categories> catList = [];
      for (var json in jsonList) {
  Categories temp = Categories.fromJson(json);
        print (temp);
        catList.add(temp);
      }
      return catList;
    } else {
      print('Response status: ${response.statusCode}');
      throw Exception("error getting categories");
    }
  }
}

model.dart

class Categories {

  String _id;
  String _name;


  Categories(String id, String name) {
    this._id = id;
    this._name = name;

  }

  factory Categories.fromJson(Map<String, dynamic> json) {
    return Categories.full(
        json['id'], json['name']);
  }
String getId() {
    return _id;
  }

  void setId(String id) {
    this._id = id;
  }

  String getName() {
    return _name;
  }

  void setName(String name) {
    this._name = name;
  }

}

category_test.dart

class MockClient extends Mock implements http.Client {}
main() {
  group('test categories', ()
  {
    test(
        'returns list of categories if the http call completes successfully', () async {
      final client = MockClient();
      when(client.get('....',


        headers: anyNamed('headers'),
      )
      ).thenAnswer((_) async {
        return http.Response(
            '{"id":"123","name":"Health"}',
            200);
      });

      List<Categories> value = await Service.customClient(client).getCats();

      expect(value, const TypeMatcher<List<Categories>>());

         expect(value[1].getId, "123");
         expect(value[1].getName, "Health");
    });
});
}

I am not exactly sure why I am getting this error.I need to write the test files and Im not sure what I am doing wrong.