diff --git a/lib/kinds/app_events.dart b/lib/kinds/app_events.dart
index 8314a65..2b28e39 100644
--- a/lib/kinds/app_events.dart
+++ b/lib/kinds/app_events.dart
@@ -3,6 +3,7 @@ import '../infra/typed_event_handler.dart';
 
 abstract class AppEvent {}
 
-abstract class AppEventHandler extends TypedEventHandler<AppEvent> {}
+abstract class AppEventHandler<A extends AppEvent>
+    extends TypedEventHandler<A> {}
 
-class AppEventBus extends TypedEventBus<AppEvent> {}
+class AppEventBus<A extends AppEvent> extends TypedEventBus<A> {}
diff --git a/lib/kinds/commands.dart b/lib/kinds/commands.dart
index 0671b9c..a156b65 100644
--- a/lib/kinds/commands.dart
+++ b/lib/kinds/commands.dart
@@ -3,6 +3,6 @@ import '../infra/typed_event_handler.dart';
 
 abstract class Command {}
 
-abstract class CommandHandler extends TypedEventHandler<Command> {}
+abstract class CommandHandler<C extends Command> extends TypedEventHandler<C> {}
 
-class CommandEventBus extends TypedEventBus<Command> {}
+class CommandEventBus<C extends Command> extends TypedEventBus<C> {}
diff --git a/lib/kinds/ui_events.dart b/lib/kinds/ui_events.dart
index ff48956..170d632 100644
--- a/lib/kinds/ui_events.dart
+++ b/lib/kinds/ui_events.dart
@@ -3,7 +3,7 @@ import '../infra/typed_event_handler.dart';
 
 abstract class UiEvent {}
 
-abstract class UIEventHandler extends TypedEventHandler<UiEvent> {}
+abstract class UiEventHandler<U extends UiEvent> extends TypedEventHandler<U> {}
 
 class UiEventBus extends TypedEventBus<UiEvent> {}