diff --git a/testcases/BinarySearchTree.scala b/testcases/BinarySearchTree.scala
index 4d5818dda411afb384b84275424f720682bcb2e3..6177499169048d7de78c2501342c500b5c8d68c1 100644
--- a/testcases/BinarySearchTree.scala
+++ b/testcases/BinarySearchTree.scala
@@ -63,7 +63,7 @@ object BinarySearchTree {
         } else {
           n
         }
-    }) ensuring(contents(_) != Set.empty[Int])
+    }) ensuring (contents(_) == contents(tree) ++ Set(value))
 
     def contains(tree: Tree, value: Int) : Boolean = tree match {
         case Leaf() => false