diff --git a/lib/game/bloc/game_event.dart b/lib/game/bloc/game_event.dart
index 7d3e3c91..0edc91ab 100644
--- a/lib/game/bloc/game_event.dart
+++ b/lib/game/bloc/game_event.dart
@@ -34,7 +34,11 @@ class Scored extends GameEvent {
 }
 
 class BonusLetterActivated extends GameEvent {
-  const BonusLetterActivated(this.letterIndex);
+  const BonusLetterActivated(this.letterIndex)
+      : assert(
+          letterIndex < GameBloc.bonusWord.length,
+          'Index must be smaller than the length of the word',
+        );
 
   final int letterIndex;
 
diff --git a/test/game/bloc/game_event_test.dart b/test/game/bloc/game_event_test.dart
index c21ecfba..d6d2278b 100644
--- a/test/game/bloc/game_event_test.dart
+++ b/test/game/bloc/game_event_test.dart
@@ -56,6 +56,16 @@ void main() {
           isNot(equals(BonusLetterActivated(1))),
         );
       });
+
+      test(
+        'throws assertion error if index is bigger than the word length',
+        () {
+          expect(
+            () => BonusLetterActivated(8),
+            throwsAssertionError,
+          );
+        },
+      );
     });
   });
 }