Nested freezed class cannot access parameter [Flutter, Freezed]

I have a bizarre issue. I am trying to generate models with a freezed package. I have nested two classes that connected each other with different key values.

This is the main class that I will use. Parameters come from data key.

@freezed
class CustomClass with _$CustomClass {
  @JsonSerializable(explicitToJson: true)
  const factory CustomClass({
    @JsonKey(name: "data.item1") required final String item1,
    @JsonKey(name: "data.item2") required final CustomClass1 item2,
  }) = _CustomClass;

  factory CustomClass.fromJson(Map<String, dynamic> json) => _$CustomClassFromJson(json);
}

This is the subclass that I will use to generate my nested model.

@freezed
class CustomClass1 with _$CustomClass1 {
  const factory CustomClass1({
    required String name,
    required num price,
  }) = _CustomClass1;

  factory CustomClass1.fromJson(Map<String, dynamic> json) => _$CustomClass1FromJson(json);
}

When I tried to model the response object data.item2, it gives the error below.

I/flutter (25343): NoSuchMethodError: The method '[]' was called on null.
I/flutter (25343): Receiver: null
I/flutter (25343): Tried calling: []("name")

Can you guys have suggestions?

Note: My response body ->

{
     "data": {
           "item1": "abcd",
           "item2": { "name": "object_name", price: 52.90 }
     },
     "error": {}
}