diff --git a/json_serializable/CHANGELOG.md b/json_serializable/CHANGELOG.md
index 849110984..7d89d80d9 100644
--- a/json_serializable/CHANGELOG.md
+++ b/json_serializable/CHANGELOG.md
@@ -1,3 +1,5 @@
+## 6.4.2-dev
+
 ## 6.4.1
 
 - Fixed a bug when an `@JsonSerializable` class uses a mixin with fields.
diff --git a/json_serializable/README.md b/json_serializable/README.md
index 44648c6dc..2d7b4c66d 100644
--- a/json_serializable/README.md
+++ b/json_serializable/README.md
@@ -167,7 +167,7 @@ For [`Map`], the key value must be one of
 If you want to use types that are not supported out-of-the-box or if you want to
 customize the encoding/decoding of any type, you have a few options.
 
-1. If you own/cotrol the desired type, add a `fromJson` constructor and/or a
+1. If you own/control the desired type, add a `fromJson` constructor and/or a
    `toJson()` function to the type. Note: while you can use `json_serializable`
    for these types, you don't have to! The generator code only looks for these
    methods. It doesn't care how they were created.
diff --git a/json_serializable/lib/src/type_helpers/map_helper.dart b/json_serializable/lib/src/type_helpers/map_helper.dart
index 8bff25263..7c99fcf5a 100644
--- a/json_serializable/lib/src/type_helpers/map_helper.dart
+++ b/json_serializable/lib/src/type_helpers/map_helper.dart
@@ -167,7 +167,7 @@ void _checkSafeKeyType(String expression, DartType keyArg) {
   throw UnsupportedTypeError(
     keyArg,
     expression,
-    'Map keys must be one of: ${_allowedTypeNames.join(', ')}.',
+    'Map keys must be one of: ${allowedMapKeyTypes.join(', ')}.',
   );
 }
 
@@ -175,12 +175,13 @@ void _checkSafeKeyType(String expression, DartType keyArg) {
 ///
 /// Used in [_checkSafeKeyType] to provide a helpful error with unsupported
 /// types.
-Iterable<String> get _allowedTypeNames => const [
+List<String> get allowedMapKeyTypes => [
       'Object',
       'dynamic',
       'enum',
       'String',
-    ].followedBy(_instances.map((i) => i.coreTypeName));
+      ..._instances.map((i) => i.coreTypeName)
+    ];
 
 extension on DartType {
   bool get isSimpleJsonTypeNotDouble =>
diff --git a/json_serializable/lib/type_helper.dart b/json_serializable/lib/type_helper.dart
index ab8a4c60a..d79a2ada7 100644
--- a/json_serializable/lib/type_helper.dart
+++ b/json_serializable/lib/type_helper.dart
@@ -14,7 +14,7 @@ export 'src/type_helpers/enum_helper.dart';
 export 'src/type_helpers/iterable_helper.dart';
 export 'src/type_helpers/json_converter_helper.dart';
 export 'src/type_helpers/json_helper.dart';
-export 'src/type_helpers/map_helper.dart';
+export 'src/type_helpers/map_helper.dart' show MapHelper;
 export 'src/type_helpers/uri_helper.dart';
 export 'src/type_helpers/value_helper.dart';
 export 'src/unsupported_type_error.dart';
diff --git a/json_serializable/pubspec.yaml b/json_serializable/pubspec.yaml
index c4eef479b..b8739a41d 100644
--- a/json_serializable/pubspec.yaml
+++ b/json_serializable/pubspec.yaml
@@ -1,5 +1,5 @@
 name: json_serializable
-version: 6.4.1
+version: 6.4.2-dev
 description: >-
   Automatically generate code for converting to and from JSON by annotating
   Dart classes.
diff --git a/json_serializable/tool/readme/readme_template.md b/json_serializable/tool/readme/readme_template.md
index 6009b06de..2f3c39551 100644
--- a/json_serializable/tool/readme/readme_template.md
+++ b/json_serializable/tool/readme/readme_template.md
@@ -103,7 +103,7 @@ For `core:Map`, the key value must be one of
 If you want to use types that are not supported out-of-the-box or if you want to
 customize the encoding/decoding of any type, you have a few options.
 
-1. If you own/cotrol the desired type, add a `fromJson` constructor and/or a
+1. If you own/control the desired type, add a `fromJson` constructor and/or a
    `toJson()` function to the type. Note: while you can use `json_serializable`
    for these types, you don't have to! The generator code only looks for these
    methods. It doesn't care how they were created.
diff --git a/json_serializable/tool/test_type_builder.dart b/json_serializable/tool/test_type_builder.dart
index bfadc7091..de9df8b2c 100644
--- a/json_serializable/tool/test_type_builder.dart
+++ b/json_serializable/tool/test_type_builder.dart
@@ -7,6 +7,7 @@ import 'dart:async';
 import 'package:build/build.dart';
 import 'package:collection/collection.dart';
 import 'package:dart_style/dart_style.dart';
+import 'package:json_serializable/src/type_helpers/map_helper.dart';
 
 import 'shared.dart';
 import 'test_type_data.dart';
@@ -96,16 +97,9 @@ final _typesToTest = {
   ..._collectionTypes,
 };
 
-const mapKeyTypes = {
-  'BigInt',
-  'DateTime',
-  'dynamic',
-  customEnumType,
-  'int',
-  'Object',
-  'String',
-  'Uri',
-};
+Iterable<String> get mapKeyTypes =>
+    allowedMapKeyTypes.map((e) => e == 'enum' ? customEnumType : e).toList()
+      ..sort(compareAsciiLowerCase);
 
 final _iterableGenericArgs = ([
   ..._trivialTypesToTest.keys,