diff --git a/pythermalcomfort/models/pmv_ppd.py b/pythermalcomfort/models/pmv_ppd.py
index dd029f2..15a1b0f 100644
--- a/pythermalcomfort/models/pmv_ppd.py
+++ b/pythermalcomfort/models/pmv_ppd.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 from typing import Union, List
 
 import numpy as np