diff --git a/sympt/solver.py b/sympt/solver.py
index 8350804..283efe3 100644
--- a/sympt/solver.py
+++ b/sympt/solver.py
@@ -841,7 +841,7 @@ def rotate(self, expr, max_order=None, return_form=None):
             The rotated expression in the specified form.
         """
         if max_order is None:
-            max_order = max(self.__S.keys())
+            max_order = self.__max_order
 
         return_form = self.__return_form if return_form is None else return_form