diff --git a/SQ-2021-H6/pom.xml b/SQ-2021-H6/pom.xml
index b2c9884..5e05f87 100644
--- a/SQ-2021-H6/pom.xml
+++ b/SQ-2021-H6/pom.xml
@@ -11,7 +11,6 @@
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <junit.platform.version>1.7.0-M1</junit.platform.version>
         <junit.jupiter.version>5.6.2</junit.jupiter.version>
-        <junit.vintage.version>5.6.2</junit.vintage.version>
         <junit.version>4.13.1</junit.version>
     </properties>
 
@@ -93,12 +92,6 @@
             <version>${junit.version}</version>
             <scope>test</scope>
         </dependency>
-        <dependency>
-			<groupId>org.junit.vintage</groupId>
-			<artifactId>junit-vintage-engine</artifactId>
-			<version>${junit.vintage.version}</version>
-			<scope>test</scope>
-		</dependency>
     </dependencies>
     <reporting>
         <plugins>
diff --git a/SQ-2021-H6/src/test/java/org/usi/sq/util/CollectionUtilsTest.java b/SQ-2021-H6/src/test/java/org/usi/sq/util/CollectionUtilsTest.java
index bebbdb7..79c1313 100644
--- a/SQ-2021-H6/src/test/java/org/usi/sq/util/CollectionUtilsTest.java
+++ b/SQ-2021-H6/src/test/java/org/usi/sq/util/CollectionUtilsTest.java
@@ -6,9 +6,9 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
 
-import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.*;
 
-class CollectionUtilsTest {
+public class CollectionUtilsTest {
 
     private static <T extends Comparable<T>> void checkSort(List<T> source, List<T> expected) {
         List<T> target = new ArrayList<>(source);
@@ -20,14 +20,44 @@ class CollectionUtilsTest {
         return Arrays.asList(elements);
     }
 
-    private static <T> ArrayList<T> empty() {
-        return new ArrayList<T>();
+    @Test
+    public void checkSorted() {
+        assertThrows(IllegalArgumentException.class, () -> CollectionUtils.sort(null));
+        checkSort(new ArrayList<Integer>(), new ArrayList<>());
+        checkSort(list(0, 1, 2, 3, 4), list(0, 1, 2, 3, 4));
+        checkSort(list(8, 4, 5, 6, 7), list(4, 5, 6, 7, 8));
+        checkSort(list(8, 7), list(7, 8));
     }
 
     @Test
-    public void checkSorted() {
-        checkSort(empty(), empty());
-        checkSort(list(0, 1, 2, 3, 4), list(0, 1, 2, 3, 4));
-        checkSort(list(8, 4, 5, 6, 7), list(4, 5, 6, 7, 8));
+    public void checkStability() {
+        class DummyAllSame implements Comparable<DummyAllSame> {
+            @Override
+            public int compareTo(DummyAllSame o) {
+                return 0; // all are same
+            }
+        }
+
+        final DummyAllSame a = new DummyAllSame();
+        final DummyAllSame b = new DummyAllSame();
+        final DummyAllSame c = new DummyAllSame();
+
+        final List<DummyAllSame> list = Arrays.asList(a, b, c);
+        CollectionUtils.sort(list);
+
+        assertSame(a, list.get(0));
+        assertSame(b, list.get(1));
+        assertSame(c, list.get(2));
+
+        assertEquals(3, list.size());
+    }
+
+    @Test
+    public void checkConstructor() {
+        try {
+            new CollectionUtils();
+        } catch (final Exception ignored) {
+            fail(ignored);
+        }
     }
 }