diff --git a/src/main/java/de/tims/tictactoe/ai/AIHard.java b/src/main/java/de/tims/tictactoe/ai/AIHard.java
index 6ea1d35..3e802c2 100644
--- a/src/main/java/de/tims/tictactoe/ai/AIHard.java
+++ b/src/main/java/de/tims/tictactoe/ai/AIHard.java
@@ -78,7 +78,11 @@ public class AIHard implements TicTacToeAI {
 		return count;
 	}
 	
-	public int countCharsInDiag(int index, char charToCount) {
+	public int countCharsInDiag(int index, char charToCount) throws IndexOutOfBoundsException {
+		if (index < 0 || index > 1) {
+			throw new IndexOutOfBoundsException("Only 0 and 1 are allowed values for index!");
+		}
+		
 		int count = 0;
 		char[][] board = gl.getBoard();
 		
diff --git a/src/test/java/de/tims/tictactoe/ai/AIHardTest.java b/src/test/java/de/tims/tictactoe/ai/AIHardTest.java
index d04e720..72934b6 100644
--- a/src/test/java/de/tims/tictactoe/ai/AIHardTest.java
+++ b/src/test/java/de/tims/tictactoe/ai/AIHardTest.java
@@ -125,5 +125,17 @@ class AIHardTest {
 						new char[][] { {'-', '-', 'o'}, {'o', 'o', '-'}, {'-', '-', '-'} },
 						1, 'o', 2));
 	}
+	
+	@Test
+	void invalidIndexCausesIndexOutOfBoundsException() {
+		int index = 2;
+		char charToCount = 'o';
+		String msg = "Only 0 and 1 are allowed values for index!";
+		doReturn(new char[][] { {'-', '-', '-'}, {'-', '-', '-'}, {'-', '-', '-'} }).when(gl).getBoard();
+		
+		AIHard ai = new AIHard(gl);
+		
+		assertThatThrownBy(() -> {ai.countCharsInDiag(index, charToCount);}).isInstanceOf(IndexOutOfBoundsException.class).hasMessage(msg);
+	}
 
 }