diff --git a/pyproject.toml b/pyproject.toml
index 3650bfb8..1fdadfda 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -188,6 +188,7 @@ target-version = ['py39', 'py310', 'py311']
 
 [tool.mypy]
 ignore_missing_imports = true  # Preferably false, but matplotlib, scipy and statsmodels are missing typing stubs
+python_version = "3.9"
 
 [tool.coverage.run]
 branch = true
diff --git a/src/dtscalibration/datastore_utils.py b/src/dtscalibration/datastore_utils.py
index 5bdbb055..fe23f53c 100644
--- a/src/dtscalibration/datastore_utils.py
+++ b/src/dtscalibration/datastore_utils.py
@@ -1,6 +1,7 @@
 # coding=utf-8
 from typing import TYPE_CHECKING
 from typing import Optional
+from typing import Union
 
 import matplotlib.pyplot as plt
 import numpy as np
@@ -13,7 +14,7 @@
 
 def check_dims(
     ds: "DataStore",
-    labels: list[str] | tuple[str],
+    labels: Union[list[str], tuple[str]],
     correct_dims: Optional[tuple[str]] = None,
 ) -> None:
     """