diff --git a/testcases/verification/datastructures/RedBlackTree.scala b/testcases/verification/datastructures/RedBlackTree.scala
index eb998becb34319e1ccfdb7230c9d314617a7854f..dec88b97b7bd23bcb726d1f3ccfa13e6409c26e3 100644
--- a/testcases/verification/datastructures/RedBlackTree.scala
+++ b/testcases/verification/datastructures/RedBlackTree.scala
@@ -14,7 +14,7 @@ object RedBlackTree {
     case Node(_, l, v, r) => content(l) ++ Set(v) ++ content(r)
   }
 
-  def size(t : Tree) : Int = t match {
+  def size(t : Tree) : BigInt = t match {
     case Empty() => 0
     case Node(_, l, v, r) => size(l) + 1 + size(r)
   }