diff --git a/pyproject.toml b/pyproject.toml
index 116e2f52..6ae6def0 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -40,7 +40,7 @@ markers = ["flaky", "network"]
 
 [tool.mypy]
 # Docs: https://mypy.readthedocs.io/en/stable/config_file.html
-python_version = 3.10
+python_version = "3.10"
 check_untyped_defs = true
 ignore_missing_imports = true
 warn_unused_ignores = true
diff --git a/xcdat/temporal.py b/xcdat/temporal.py
index a0daf9a4..05e7db57 100644
--- a/xcdat/temporal.py
+++ b/xcdat/temporal.py
@@ -732,7 +732,7 @@ def departures(
             # The original time dimension name is restored after grouped
             # arithmetic, so the labeled time dimension name is no longer needed
             # and therefore dropped.
-            ds_obs = ds_obs.drop_vars(self._labeled_time.name)
+            ds_obs = ds_obs.drop_vars(str(self._labeled_time.name))
 
         if weighted and keep_weights:
             self._weights = ds_climo.time_wts