diff --git a/src/calliope/core/io.py b/src/calliope/core/io.py
index 8542db88d..5a8ccbf02 100644
--- a/src/calliope/core/io.py
+++ b/src/calliope/core/io.py
@@ -10,6 +10,7 @@
 """
 
 import os
+from typing import Union
 
 # We import netCDF4 before xarray to mitigate a numpy warning:
 # https://github.com/pydata/xarray/issues/7259