diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index be05b70..76d1a63 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -2,6 +2,7 @@ name: CI
 
 on:
   push:
+  pull_request:
 
 jobs:
   test:
diff --git a/src/main/java/io/github/vbetsch/yams/InvalidRollSizeError.java b/src/main/java/io/github/vbetsch/yams/InvalidRollSizeError.java
new file mode 100644
index 0000000..cc71e7c
--- /dev/null
+++ b/src/main/java/io/github/vbetsch/yams/InvalidRollSizeError.java
@@ -0,0 +1,7 @@
+package io.github.vbetsch.yams;
+
+public class InvalidRollSizeError extends IllegalArgumentException {
+    public InvalidRollSizeError(int actualSize) {
+        super("Roll must contain exactly " + Yams.DICES_AMOUNT + " dices but got " + actualSize);
+    }
+}
diff --git a/src/main/java/io/github/vbetsch/yams/Yams.java b/src/main/java/io/github/vbetsch/yams/Yams.java
index e32f60c..cf83cf0 100644
--- a/src/main/java/io/github/vbetsch/yams/Yams.java
+++ b/src/main/java/io/github/vbetsch/yams/Yams.java
@@ -4,7 +4,7 @@
 import java.util.stream.IntStream;
 
 public class Yams {
-    private final int DICES_AMOUNT = 5;
+    static final int DICES_AMOUNT = 5;
 
     private int calculateSumOfDices(List<Integer> roll) {
         return roll
@@ -43,7 +43,7 @@ private boolean containsDuplicatesNth(List<Integer> roll, int duplicatesNumber)
                 .stream()
                 .mapToInt(Integer::intValue)
                 .distinct();
-        return reducedRoll.count() == (this.DICES_AMOUNT - duplicatesNumber + 1);
+        return reducedRoll.count() == (Yams.DICES_AMOUNT - duplicatesNumber + 1);
     }
 
     private int handleThreeOfAKindScore(List<Integer> roll) throws IllegalArgumentException {
@@ -60,7 +60,10 @@ private int handleFourOfAKindScore(List<Integer> roll) throws IllegalArgumentExc
         return this.calculateSumOfDices(roll);
     }
 
-    public int computeScore(List<Integer> roll, CategoryEnum category) {
+    public int computeScore(List<Integer> roll, CategoryEnum category) throws InvalidRollSizeError {
+        if(roll.size() != Yams.DICES_AMOUNT) {
+            throw new InvalidRollSizeError(roll.size());
+        }
         return switch (category) {
             case CategoryEnum.CHANCE -> this.handleChanceScore(roll);
             case CategoryEnum.YAMS -> this.handleYamsScore(roll);
diff --git a/src/test/java/io/github/vbetsch/yams/YamsTest.java b/src/test/java/io/github/vbetsch/yams/YamsTest.java
index 30c58d1..b12d4c9 100644
--- a/src/test/java/io/github/vbetsch/yams/YamsTest.java
+++ b/src/test/java/io/github/vbetsch/yams/YamsTest.java
@@ -225,4 +225,46 @@ void fourOfAKind_returnIllegalArgumentException_whenGivenLargeStraight() {
                 "We cannot compute score with category FourOfAKind for this roll"
         );
     }
+
+    @Test
+    void shouldThrowInvalidRollSizeErrorWhenGivenLessThanFiveDices() {
+        // Arrange
+        Yams yams = new Yams();
+        List<Integer> roll = List.of(1, 2, 3, 4);
+
+        // Act & Assert
+        assertThrows(
+                InvalidRollSizeError.class,
+                () -> yams.computeScore(roll, CategoryEnum.CHANCE),
+                "Roll must contain exactly 5 dices"
+        );
+    }
+
+    @Test
+    void shouldThrowInvalidRollSizeErrorWhenGivenMoreThanFiveDices() {
+        // Arrange
+        Yams yams = new Yams();
+        List<Integer> roll = List.of(1, 2, 3, 4, 5, 6);
+
+        // Act & Assert
+        assertThrows(
+                InvalidRollSizeError.class,
+                () -> yams.computeScore(roll, CategoryEnum.CHANCE),
+                "Roll must contain exactly 5 dices"
+        );
+    }
+
+    @Test
+    void shouldThrowInvalidRollSizeErrorWhenGivenNoDice() {
+        // Arrange
+        Yams yams = new Yams();
+        List<Integer> roll = List.of();
+
+        // Act & Assert
+        assertThrows(
+                InvalidRollSizeError.class,
+                () -> yams.computeScore(roll, CategoryEnum.CHANCE),
+                "Roll must contain exactly 5 dices"
+        );
+    }
 }