json_serializable what am I overlooking? - json

Question

I'm having trouble with jsonEncode even though I've done this multiple times and followed the documentation (Link). I'm not seeing what is wrong.
import 'package:json_annotation/json_annotation.dart';
part 'block.g.dart';
#JsonSerializable()
class Block {
Block(
this.name,
this.required,
this.type,
this.stringValue,
this.checkboxValue,
this.numericValue,
this.minNumericRestraint,
this.maxNumericRestraint
);
String name;
bool required;
String type;
String stringValue;
bool checkboxValue;
double numericValue;
double minNumericRestraint;
double maxNumericRestraint;
factory Block.fromJson(Map<String, dynamic> json) => _$BlockFromJson(json);
Map<String, dynamic> toJson() => _$BlockToJson(this);
}
// GENERATED CODE - DO NOT MODIFY BY HAND
part of 'block.dart';
// **************************************************************************
// JsonSerializableGenerator
// **************************************************************************
Block _$BlockFromJson(Map<String, dynamic> json) => Block(
json['name'] as String,
json['required'] as bool,
json['type'] as String,
json['stringValue'] as String,
json['checkboxValue'] as bool,
(json['numericValue'] as num).toDouble(),
(json['minNumericRestraint'] as num).toDouble(),
(json['maxNumericRestraint'] as num).toDouble(),
);
Map<String, dynamic> _$BlockToJson(Block instance) => <String, dynamic>{
'name': instance.name,
'required': instance.required,
'type': instance.type,
'stringValue': instance.stringValue,
'checkboxValue': instance.checkboxValue,
'numericValue': instance.numericValue,
'minNumericRestraint': instance.minNumericRestraint,
'maxNumericRestraint': instance.maxNumericRestraint,
}
Calling jsonEncode(Block(...)); results in following error: Error: Converting object to an encodable object failed: Instance of 'Block'
I reran the build script multiple times, wrote the toJson() method manually and invalidated my Android Studio cache.