diff --git a/src/HierarchicalSystem.java b/src/HierarchicalSystem.java
index 88cb36c..46faa55 100644
--- a/src/HierarchicalSystem.java
+++ b/src/HierarchicalSystem.java
@@ -1,5 +1,7 @@
 import codedraw.CodeDraw;
 
+import java.util.NoSuchElementException;
+
 /**
  * A cosmic system that is composed of a central named body (of type 'NamedBodyForcePair')
  * and an arbitrary number of subsystems (of type 'CosmicSystem') in its orbit.
@@ -136,6 +138,7 @@ public class HierarchicalSystem implements CosmicSystem, MassiveIterable {
 
             @Override
             public Massive next() {
+                if (!hasNext()) throw new NoSuchElementException();
                 if (cur != null && cur.hasNext()) return cur.next();
 
                 while (i < all.length) {
diff --git a/src/MassiveForceTreeMap.java b/src/MassiveForceTreeMap.java
index 8b3036e..523fdb1 100644
--- a/src/MassiveForceTreeMap.java
+++ b/src/MassiveForceTreeMap.java
@@ -1,5 +1,7 @@
 import codedraw.CodeDraw;
 
+import java.util.NoSuchElementException;
+
 /**
  * A map that associates an object of 'Massive' with a Vector3. The number of key-value pairs
  * is not limited.
@@ -124,7 +126,8 @@ public class MassiveForceTreeMap implements MassiveSet {
 
             @Override
             public Massive next() {
-                if (next == null) return null;
+                if (!hasNext()) throw new NoSuchElementException();
+
                 Massive m = next.key;
                 Item newNext = (next.right != null) ? next.right.getLeftLeaf() : next.parent;
                 while (newNext != null && newNext.right == next) {
diff --git a/src/MassiveLinkedList.java b/src/MassiveLinkedList.java
index ebe08c0..31e97d9 100644
--- a/src/MassiveLinkedList.java
+++ b/src/MassiveLinkedList.java
@@ -1,4 +1,5 @@
 import java.util.Iterator;
+import java.util.NoSuchElementException;
 
 /**
  * A list of massive objects implemented as a linked list.
@@ -184,6 +185,7 @@ public class MassiveLinkedList implements MassiveIterable {
 
             @Override
             public Massive next() {
+                if (!hasNext()) throw new NoSuchElementException();
                 if (!yieldedFirst) {
                     yieldedFirst = true;
                 } else {