diff --git a/wavl.js b/wavl.js
index 121ae1d..3b240c1 100644
--- a/wavl.js
+++ b/wavl.js
@@ -5,7 +5,7 @@ class WAVLTree extends AVLTree {
         }
 
         nodeDifferences(node).forEach((childRank) => {
-            if ([1, 2].findIndex(childRank) == -1) {
+            if ([1, 2].indexOf(childRank) == -1) {
                 return false;
             }
         });