Flutter Freezed: generated code not calling toJson() on object from another package

Question

I have a freezed class with the following fields:

import 'package:freezed_annotation/freezed_annotation.dart';
import 'package:latlong2/latlong.dart';

part 'event.freezed.dart';
part 'event.g.dart';

@freezed
class Event with _$Event {
  const factory Event({
    required LatLng location,
    required DateTime date,
    required String name,
  }) = _Event;

  factory Event.fromJson(Map<String, Object?> json) => _$EventFromJson(json);
}

Notice that the LatLng object comes from a package called latlong2. When I generate the code with build runner I get the following output.

// GENERATED CODE - DO NOT MODIFY BY HAND

part of 'event.dart';

// **************************************************************************
// JsonSerializableGenerator
// **************************************************************************

_$_Event _$$_EventFromJson(Map<String, dynamic> json) => _$_Event(
      location: LatLng.fromJson(json['location'] as Map<String, dynamic>),
      date: DateTime.parse(json['date'] as String),
      name: json['name'] as String,
    );

Map<String, dynamic> _$$_EventToJson(_$_Event instance) => <String, dynamic>{
      'location': instance.location,
      'date': instance.date.toIso8601String(),
      'name': instance.name,
    };

For some reason json_serializable properly decodes the object but is unable to encode it because it doesn't call toJson() on instance.location. How did I fix this?