diff --git a/src/lib/game.ts b/src/lib/game.ts
index 5c0a38f..e500dcf 100644
--- a/src/lib/game.ts
+++ b/src/lib/game.ts
@@ -24,7 +24,7 @@ export const DefaultRandom:RandomSource = {
 }
 
 export interface ActionFactory {
-  next(game: Game): Action
+  next(game: Game): Action | undefined
 }
 
 // an action is in charge of the entire state transition