diff --git a/pyTMD/math.py b/pyTMD/math.py
index 3206db6..ae4613a 100644
--- a/pyTMD/math.py
+++ b/pyTMD/math.py
@@ -15,6 +15,8 @@
     Updated 12/2024: added function to calculate an aliasing frequency
     Written 11/2024
 """
+from __future__ import annotations
+
 import numpy as np
 from scipy.special import factorial