diff --git a/lib/src/typedefs.dart b/lib/src/typedefs.dart
index fd49f18..c6582d1 100644
--- a/lib/src/typedefs.dart
+++ b/lib/src/typedefs.dart
@@ -9,7 +9,7 @@ typedef CardSwiperOnSwipe = FutureOr<bool> Function(
   CardSwiperDirection direction,
 );
 
-typedef CardSwiperOnSwipeUpdate = Function(
+typedef CardSwiperOnSwipeUpdate = void Function(
   int? currentIndex,
   CardSwiperDirection direction,
 );
@@ -21,7 +21,7 @@ typedef NullableCardBuilder = Widget? Function(
   int verticalOffsetPercentage,
 );
 
-typedef CardSwiperDirectionChange = Function(
+typedef CardSwiperDirectionChange = void Function(
   CardSwiperDirection horizontalDirection,
   CardSwiperDirection verticalDirection,
 );
diff --git a/lib/src/undoable.dart b/lib/src/undoable.dart
index f79f428..888c3d9 100644
--- a/lib/src/undoable.dart
+++ b/lib/src/undoable.dart
@@ -1,8 +1,8 @@
 class Undoable<T> {
-  Undoable(this._value, {Undoable? previousValue}) : _previous = previousValue;
+  Undoable(this._value, {Undoable<T>? previousValue}) : _previous = previousValue;
 
   T _value;
-  Undoable? _previous;
+  Undoable<T>? _previous;
 
   T get state => _value;
   T? get previousState => _previous?.state;