diff --git a/src/core/MancalaGame.ts b/src/core/MancalaGame.ts
index d613bf2..75a42d3 100644
--- a/src/core/MancalaGame.ts
+++ b/src/core/MancalaGame.ts
@@ -198,17 +198,18 @@ export class MancalaGame {
     return false;
   }
 
-  public getWonPlayerId(): string | null {
+  public getWonPlayerId(): string | undefined {
+    const player1StoneCount = this.board.player1Bank.stoneCount;
+    const player2StoneCount = this.board.player2Bank.stoneCount;
     if (this.checkGameIsEnded()) {
-      if (
-        this.board.player1Bank.stoneCount > this.board.player2Bank.stoneCount
-      ) {
+      if (player1StoneCount === player2StoneCount) {
+        return undefined;
+      } else if (player1StoneCount > player2StoneCount) {
         return this.player1Id;
       } else {
         return this.player2Id;
       }
     }
-    return null;
   }
 
   public static createFromMancalaGame(mancalaGame: MancalaGame): MancalaGame {
diff --git a/tests/MancalaGame.test.ts b/tests/MancalaGame.test.ts
index e8c39d9..4073244 100644
--- a/tests/MancalaGame.test.ts
+++ b/tests/MancalaGame.test.ts
@@ -153,4 +153,24 @@ describe('Game Test', () => {
     expect(game.history[2]).toStrictEqual(historyItem3);
     expect(game.history[3]).toStrictEqual(historyItem4);
   });
+
+  test('test getWonPlayerId', () => {
+    const game = createGame();
+    const player1Id = '0';
+    const player2Id = '1';
+
+    game.board.clear();
+
+    game.board.pits[game.board.player1BankIndex()].stoneCount = 1;
+    game.board.pits[game.board.player2BankIndex()].stoneCount = 0;
+    expect(game.getWonPlayerId()).toBe(player1Id);
+
+    game.board.pits[game.board.player1BankIndex()].stoneCount = 0;
+    game.board.pits[game.board.player2BankIndex()].stoneCount = 1;
+    expect(game.getWonPlayerId()).toBe(player2Id);
+
+    game.board.pits[game.board.player1BankIndex()].stoneCount = 0;
+    game.board.pits[game.board.player2BankIndex()].stoneCount = 0;
+    expect(game.getWonPlayerId()).toBe(undefined);
+  });
 });