diff --git a/avl.py b/avl.py
index 5445168..eda3a63 100644
--- a/avl.py
+++ b/avl.py
@@ -29,7 +29,11 @@ class AVLTree(RankedTree[T]):
         if not node:
             return True
 
-        if not (-1 <= _balance_factor(node) <= 1):
+        if not (
+            -1 <= _balance_factor(node) <= 1
+            and node.rank
+            == 1 + max(Node.get_rank(node.left), Node.get_rank(node.right))
+        ):
             return False
 
         return not recursive or (