diff --git a/lib/src/messages/abort.dart b/lib/src/messages/abort.dart
index 5eb0f8f..f1b0604 100644
--- a/lib/src/messages/abort.dart
+++ b/lib/src/messages/abort.dart
@@ -7,9 +7,9 @@ abstract class IAbortFields {
 
   String get reason;
 
-  List<dynamic> get args;
+  List<dynamic>? get args;
 
-  Map<String, dynamic> get kwargs;
+  Map<String, dynamic>? get kwargs;
 }
 
 class AbortFields implements IAbortFields {
@@ -18,14 +18,14 @@ class AbortFields implements IAbortFields {
     this._reason, {
     List<dynamic>? args,
     Map<String, dynamic>? kwargs,
-  })  : _args = args ?? [],
-        _kwargs = kwargs ?? {};
+  })  : _args = args,
+        _kwargs = kwargs;
 
   final Map<String, dynamic> _details;
   final String _reason;
 
-  final List<dynamic> _args;
-  final Map<String, dynamic> _kwargs;
+  final List<dynamic>? _args;
+  final Map<String, dynamic>? _kwargs;
 
   @override
   Map<String, dynamic> get details => _details;
@@ -34,10 +34,10 @@ class AbortFields implements IAbortFields {
   String get reason => _reason;
 
   @override
-  List get args => _args;
+  List? get args => _args;
 
   @override
-  Map<String, dynamic> get kwargs => _kwargs;
+  Map<String, dynamic>? get kwargs => _kwargs;
 }
 
 class Abort implements Message {
diff --git a/lib/src/messages/call.dart b/lib/src/messages/call.dart
index 277fd35..3a2e371 100644
--- a/lib/src/messages/call.dart
+++ b/lib/src/messages/call.dart
@@ -7,9 +7,9 @@ abstract class ICallFields {
 
   String get uri;
 
-  List<dynamic> get args;
+  List<dynamic>? get args;
 
-  Map<String, dynamic> get kwargs;
+  Map<String, dynamic>? get kwargs;
 
   Map<String, dynamic> get options;
 }
@@ -21,14 +21,14 @@ class CallFields implements ICallFields {
     List<dynamic>? args,
     Map<String, dynamic>? kwargs,
     Map<String, dynamic>? options,
-  })  : _args = args ?? [],
-        _kwargs = kwargs ?? {},
+  })  : _args = args,
+        _kwargs = kwargs,
         _options = options ?? {};
 
   final int _requestID;
   final String _uri;
-  final List<dynamic> _args;
-  final Map<String, dynamic> _kwargs;
+  final List<dynamic>? _args;
+  final Map<String, dynamic>? _kwargs;
   final Map<String, dynamic> _options;
 
   @override
@@ -38,10 +38,10 @@ class CallFields implements ICallFields {
   String get uri => _uri;
 
   @override
-  List get args => _args;
+  List? get args => _args;
 
   @override
-  Map<String, dynamic> get kwargs => _kwargs;
+  Map<String, dynamic>? get kwargs => _kwargs;
 
   @override
   Map<String, dynamic> get options => _options;
@@ -77,9 +77,9 @@ class Call implements Message {
 
   String get uri => _callFields.uri;
 
-  List<dynamic> get args => _callFields.args;
+  List<dynamic>? get args => _callFields.args;
 
-  Map<String, dynamic> get kwargs => _callFields.kwargs;
+  Map<String, dynamic>? get kwargs => _callFields.kwargs;
 
   Map<String, dynamic> get options => _callFields.options;
 
@@ -92,12 +92,12 @@ class Call implements Message {
   @override
   List<dynamic> marshal() {
     List<dynamic> message = [id, requestID, options, uri];
-    if (args.isNotEmpty) {
+    if (args != null) {
       message.add(args);
     }
 
-    if (kwargs.isNotEmpty) {
-      if (args.isEmpty) {
+    if (kwargs != null) {
+      if (args == null) {
         message.add([]);
       }
       message.add(kwargs);
diff --git a/lib/src/messages/error.dart b/lib/src/messages/error.dart
index cc15af3..93fe3a7 100644
--- a/lib/src/messages/error.dart
+++ b/lib/src/messages/error.dart
@@ -9,9 +9,9 @@ abstract class IErrorFields {
 
   String get uri;
 
-  List<dynamic> get args;
+  List<dynamic>? get args;
 
-  Map<String, dynamic> get kwargs;
+  Map<String, dynamic>? get kwargs;
 
   Map<String, dynamic> get details;
 }
@@ -24,15 +24,15 @@ class ErrorFields implements IErrorFields {
     List<dynamic>? args,
     Map<String, dynamic>? kwargs,
     Map<String, dynamic>? details,
-  })  : _args = args ?? [],
-        _kwargs = kwargs ?? {},
+  })  : _args = args,
+        _kwargs = kwargs,
         _details = details ?? {};
 
   final int _msgType;
   final int _requestID;
   final String _uri;
-  final List<dynamic> _args;
-  final Map<String, dynamic> _kwargs;
+  final List<dynamic>? _args;
+  final Map<String, dynamic>? _kwargs;
   final Map<String, dynamic> _details;
 
   @override
@@ -45,10 +45,10 @@ class ErrorFields implements IErrorFields {
   String get uri => _uri;
 
   @override
-  List get args => _args;
+  List? get args => _args;
 
   @override
-  Map<String, dynamic> get kwargs => _kwargs;
+  Map<String, dynamic>? get kwargs => _kwargs;
 
   @override
   Map<String, dynamic> get details => _details;
@@ -94,9 +94,9 @@ class Error implements Message {
 
   String get uri => _errorFields.uri;
 
-  List<dynamic> get args => _errorFields.args;
+  List<dynamic>? get args => _errorFields.args;
 
-  Map<String, dynamic> get kwargs => _errorFields.kwargs;
+  Map<String, dynamic>? get kwargs => _errorFields.kwargs;
 
   Map<String, dynamic> get details => _errorFields.details;
 
@@ -116,12 +116,12 @@ class Error implements Message {
   @override
   List<dynamic> marshal() {
     List<dynamic> message = [id, msgType, requestID, details, uri];
-    if (args.isNotEmpty) {
+    if (args != null) {
       message.add(args);
     }
 
-    if (kwargs.isNotEmpty) {
-      if (args.isEmpty) {
+    if (kwargs != null) {
+      if (args == null) {
         message.add([]);
       }
       message.add(kwargs);
diff --git a/lib/src/messages/event.dart b/lib/src/messages/event.dart
index 7b4ecad..4b7d52d 100644
--- a/lib/src/messages/event.dart
+++ b/lib/src/messages/event.dart
@@ -7,9 +7,9 @@ abstract class IEventFields {
 
   int get publicationID;
 
-  List<dynamic> get args;
+  List<dynamic>? get args;
 
-  Map<String, dynamic> get kwargs;
+  Map<String, dynamic>? get kwargs;
 
   Map<String, dynamic> get details;
 }
@@ -21,14 +21,14 @@ class EventFields implements IEventFields {
     List<dynamic>? args,
     Map<String, dynamic>? kwargs,
     Map<String, dynamic>? details,
-  })  : _args = args ?? [],
-        _kwargs = kwargs ?? {},
+  })  : _args = args,
+        _kwargs = kwargs,
         _details = details ?? {};
 
   final int _subscriptionID;
   final int _publicationID;
-  final List<dynamic> _args;
-  final Map<String, dynamic> _kwargs;
+  final List<dynamic>? _args;
+  final Map<String, dynamic>? _kwargs;
   final Map<String, dynamic> _details;
 
   @override
@@ -38,10 +38,10 @@ class EventFields implements IEventFields {
   int get publicationID => _publicationID;
 
   @override
-  List get args => _args;
+  List? get args => _args;
 
   @override
-  Map<String, dynamic> get kwargs => _kwargs;
+  Map<String, dynamic>? get kwargs => _kwargs;
 
   @override
   Map<String, dynamic> get details => _details;
@@ -83,9 +83,9 @@ class Event implements Message {
 
   int get publicationID => _eventFields.publicationID;
 
-  List<dynamic> get args => _eventFields.args;
+  List<dynamic>? get args => _eventFields.args;
 
-  Map<String, dynamic> get kwargs => _eventFields.kwargs;
+  Map<String, dynamic>? get kwargs => _eventFields.kwargs;
 
   Map<String, dynamic> get details => _eventFields.details;
 
@@ -104,12 +104,12 @@ class Event implements Message {
   @override
   List<dynamic> marshal() {
     List<dynamic> message = [id, subscriptionID, publicationID, details];
-    if (args.isNotEmpty) {
+    if (args != null) {
       message.add(args);
     }
 
-    if (kwargs.isNotEmpty) {
-      if (args.isEmpty) {
+    if (kwargs != null) {
+      if (args == null) {
         message.add([]);
       }
       message.add(kwargs);
diff --git a/lib/src/messages/invocation.dart b/lib/src/messages/invocation.dart
index dec9622..1f785f9 100644
--- a/lib/src/messages/invocation.dart
+++ b/lib/src/messages/invocation.dart
@@ -7,9 +7,9 @@ abstract class IInvocationFields {
 
   int get registrationID;
 
-  List<dynamic> get args;
+  List<dynamic>? get args;
 
-  Map<String, dynamic> get kwargs;
+  Map<String, dynamic>? get kwargs;
 
   Map<String, dynamic> get details;
 }
@@ -21,14 +21,14 @@ class InvocationFields implements IInvocationFields {
     List<dynamic>? args,
     Map<String, dynamic>? kwargs,
     Map<String, dynamic>? details,
-  })  : _args = args ?? [],
-        _kwargs = kwargs ?? {},
+  })  : _args = args,
+        _kwargs = kwargs,
         _details = details ?? {};
 
   final int _requestID;
   final int _registrationID;
-  final List<dynamic> _args;
-  final Map<String, dynamic> _kwargs;
+  final List<dynamic>? _args;
+  final Map<String, dynamic>? _kwargs;
   final Map<String, dynamic> _details;
 
   @override
@@ -38,10 +38,10 @@ class InvocationFields implements IInvocationFields {
   int get registrationID => _registrationID;
 
   @override
-  List get args => _args;
+  List? get args => _args;
 
   @override
-  Map<String, dynamic> get kwargs => _kwargs;
+  Map<String, dynamic>? get kwargs => _kwargs;
 
   @override
   Map<String, dynamic> get details => _details;
@@ -83,9 +83,9 @@ class Invocation implements Message {
 
   int get registrationID => _invocationFields.registrationID;
 
-  List<dynamic> get args => _invocationFields.args;
+  List<dynamic>? get args => _invocationFields.args;
 
-  Map<String, dynamic> get kwargs => _invocationFields.kwargs;
+  Map<String, dynamic>? get kwargs => _invocationFields.kwargs;
 
   Map<String, dynamic> get details => _invocationFields.details;
 
@@ -104,12 +104,12 @@ class Invocation implements Message {
   @override
   List<dynamic> marshal() {
     List<dynamic> message = [id, requestID, registrationID, details];
-    if (args.isNotEmpty) {
+    if (args != null) {
       message.add(args);
     }
 
-    if (kwargs.isNotEmpty) {
-      if (args.isEmpty) {
+    if (kwargs != null) {
+      if (args == null) {
         message.add([]);
       }
       message.add(kwargs);
diff --git a/lib/src/messages/publish.dart b/lib/src/messages/publish.dart
index 87123fe..8321e73 100644
--- a/lib/src/messages/publish.dart
+++ b/lib/src/messages/publish.dart
@@ -7,9 +7,9 @@ abstract class IPublishFields {
 
   String get uri;
 
-  List<dynamic> get args;
+  List<dynamic>? get args;
 
-  Map<String, dynamic> get kwargs;
+  Map<String, dynamic>? get kwargs;
 
   Map<String, dynamic> get options;
 }
@@ -21,14 +21,14 @@ class PublishFields implements IPublishFields {
     List<dynamic>? args,
     Map<String, dynamic>? kwargs,
     Map<String, dynamic>? options,
-  })  : _args = args ?? [],
-        _kwargs = kwargs ?? {},
+  })  : _args = args,
+        _kwargs = kwargs,
         _options = options ?? {};
 
   final int _requestID;
   final String _uri;
-  final List<dynamic> _args;
-  final Map<String, dynamic> _kwargs;
+  final List<dynamic>? _args;
+  final Map<String, dynamic>? _kwargs;
   final Map<String, dynamic> _options;
 
   @override
@@ -38,10 +38,10 @@ class PublishFields implements IPublishFields {
   String get uri => _uri;
 
   @override
-  List get args => _args;
+  List? get args => _args;
 
   @override
-  Map<String, dynamic> get kwargs => _kwargs;
+  Map<String, dynamic>? get kwargs => _kwargs;
 
   @override
   Map<String, dynamic> get options => _options;
@@ -83,9 +83,9 @@ class Publish implements Message {
 
   String get uri => _publishFields.uri;
 
-  List<dynamic> get args => _publishFields.args;
+  List<dynamic>? get args => _publishFields.args;
 
-  Map<String, dynamic> get kwargs => _publishFields.kwargs;
+  Map<String, dynamic>? get kwargs => _publishFields.kwargs;
 
   Map<String, dynamic> get options => _publishFields.options;
 
@@ -98,12 +98,12 @@ class Publish implements Message {
   @override
   List<dynamic> marshal() {
     List<dynamic> message = [id, requestID, options, uri];
-    if (args.isNotEmpty) {
+    if (args != null) {
       message.add(args);
     }
 
-    if (kwargs.isNotEmpty) {
-      if (args.isEmpty) {
+    if (kwargs != null) {
+      if (args == null) {
         message.add([]);
       }
       message.add(kwargs);
diff --git a/lib/src/messages/result.dart b/lib/src/messages/result.dart
index d052d71..c9e3458 100644
--- a/lib/src/messages/result.dart
+++ b/lib/src/messages/result.dart
@@ -5,9 +5,9 @@ import "package:wampproto/src/messages/validation_spec.dart";
 abstract class IResultFields {
   int get requestID;
 
-  List<dynamic> get args;
+  List<dynamic>? get args;
 
-  Map<String, dynamic> get kwargs;
+  Map<String, dynamic>? get kwargs;
 
   Map<String, dynamic> get details;
 }
@@ -18,23 +18,23 @@ class ResultFields implements IResultFields {
     List<dynamic>? args,
     Map<String, dynamic>? kwargs,
     Map<String, dynamic>? details,
-  })  : _args = args ?? [],
-        _kwargs = kwargs ?? {},
+  })  : _args = args,
+        _kwargs = kwargs,
         _details = details ?? {};
 
   final int _requestID;
-  final List<dynamic> _args;
-  final Map<String, dynamic> _kwargs;
+  final List<dynamic>? _args;
+  final Map<String, dynamic>? _kwargs;
   final Map<String, dynamic> _details;
 
   @override
   int get requestID => _requestID;
 
   @override
-  List get args => _args;
+  List? get args => _args;
 
   @override
-  Map<String, dynamic> get kwargs => _kwargs;
+  Map<String, dynamic>? get kwargs => _kwargs;
 
   @override
   Map<String, dynamic> get details => _details;
@@ -67,9 +67,9 @@ class Result implements Message {
 
   int get requestID => _resultFields.requestID;
 
-  List<dynamic> get args => _resultFields.args;
+  List<dynamic>? get args => _resultFields.args;
 
-  Map<String, dynamic> get kwargs => _resultFields.kwargs;
+  Map<String, dynamic>? get kwargs => _resultFields.kwargs;
 
   Map<String, dynamic> get details => _resultFields.details;
 
@@ -82,12 +82,12 @@ class Result implements Message {
   @override
   List<dynamic> marshal() {
     List<dynamic> message = [id, requestID, details];
-    if (args.isNotEmpty) {
+    if (args != null) {
       message.add(args);
     }
 
-    if (kwargs.isNotEmpty) {
-      if (args.isEmpty) {
+    if (kwargs != null) {
+      if (args == null) {
         message.add([]);
       }
       message.add(kwargs);
diff --git a/lib/src/messages/yield.dart b/lib/src/messages/yield.dart
index 27aaa51..d08711f 100644
--- a/lib/src/messages/yield.dart
+++ b/lib/src/messages/yield.dart
@@ -5,9 +5,9 @@ import "package:wampproto/src/messages/validation_spec.dart";
 abstract class IYieldFields {
   int get requestID;
 
-  List<dynamic> get args;
+  List<dynamic>? get args;
 
-  Map<String, dynamic> get kwargs;
+  Map<String, dynamic>? get kwargs;
 
   Map<String, dynamic> get options;
 }
@@ -18,23 +18,23 @@ class YieldFields implements IYieldFields {
     List<dynamic>? args,
     Map<String, dynamic>? kwargs,
     Map<String, dynamic>? options,
-  })  : _args = args ?? [],
-        _kwargs = kwargs ?? {},
+  })  : _args = args,
+        _kwargs = kwargs,
         _options = options ?? {};
 
   final int _requestID;
-  final List<dynamic> _args;
-  final Map<String, dynamic> _kwargs;
+  final List<dynamic>? _args;
+  final Map<String, dynamic>? _kwargs;
   final Map<String, dynamic> _options;
 
   @override
   int get requestID => _requestID;
 
   @override
-  List get args => _args;
+  List? get args => _args;
 
   @override
-  Map<String, dynamic> get kwargs => _kwargs;
+  Map<String, dynamic>? get kwargs => _kwargs;
 
   @override
   Map<String, dynamic> get options => _options;
@@ -67,9 +67,9 @@ class Yield implements Message {
 
   int get requestID => _yieldFields.requestID;
 
-  List<dynamic> get args => _yieldFields.args;
+  List<dynamic>? get args => _yieldFields.args;
 
-  Map<String, dynamic> get kwargs => _yieldFields.kwargs;
+  Map<String, dynamic>? get kwargs => _yieldFields.kwargs;
 
   Map<String, dynamic> get options => _yieldFields.options;
 
@@ -82,12 +82,12 @@ class Yield implements Message {
   @override
   List<dynamic> marshal() {
     List<dynamic> message = [id, requestID, options];
-    if (args.isNotEmpty) {
+    if (args != null) {
       message.add(args);
     }
 
-    if (kwargs.isNotEmpty) {
-      if (args.isEmpty) {
+    if (kwargs != null) {
+      if (args == null) {
         message.add([]);
       }
       message.add(kwargs);