diff --git a/app/src/main/java/ch/nuth/zhaw/exbox/AVLSearchTree.java b/app/src/main/java/ch/nuth/zhaw/exbox/AVLSearchTree.java
index 34be94c..e6d6dc1 100644
--- a/app/src/main/java/ch/nuth/zhaw/exbox/AVLSearchTree.java
+++ b/app/src/main/java/ch/nuth/zhaw/exbox/AVLSearchTree.java
@@ -24,7 +24,11 @@ public class AVLSearchTree<T extends Comparable<T>> extends SortedBinaryTree<T>
 
     @Override
     protected int calcSize(TreeNode<T> p) {
-        return super.calcSize(p) + p.values.size() - 1;
+        if (p == null) {
+            return 0;
+        } else {
+            return super.calcSize(p) + p.values.size() - 1;
+        }
     }
 
     /**