diff --git a/src/mrds/trees/heighted.rs b/src/mrds/trees/heighted.rs
index b6820d1..fafbd2b 100644
--- a/src/mrds/trees/heighted.rs
+++ b/src/mrds/trees/heighted.rs
@@ -165,6 +165,10 @@ impl<'a, A: 'a + Ord + Clone> BinaryTreesUnbalanced<'a> for Trees<A> {
     fn balancing_error<T: 'a>(&self, _error: BalancingError) -> BTWrap<'a, Self, T> {
         panic!("balancing error.")
     }
+
+    fn node_heights(&self, node: &Self::Node) -> (u64, u64) {
+        (node.l.height, node.r.height)
+    }
 }
 
 impl<'a, A: 'a + Ord + Clone> BinaryTreesBindable<'a> for Trees<A> {