diff --git a/corrlib/meas_io.py b/corrlib/meas_io.py
index 2a8c986..2f08052 100644
--- a/corrlib/meas_io.py
+++ b/corrlib/meas_io.py
@@ -12,7 +12,7 @@ import shutil
 from typing import Any
 
 
-def write_measurement(path: str, ensemble: str, measurement: dict[str, dict[str, dict[str, Any]]], uuid: str, code: str, parameter_file: Union(Any, None)) -> None:
+def write_measurement(path: str, ensemble: str, measurement: dict[str, dict[str, dict[str, Any]]], uuid: str, code: str, parameter_file: Union[Any, None]) -> None:
     """
     Write a measurement to the backlog.
     If the file for the measurement already exists, update the measurement.