diff --git a/nion/swift/model/Utility.py b/nion/swift/model/Utility.py
index ea6b574df..364f0ab07 100644
--- a/nion/swift/model/Utility.py
+++ b/nion/swift/model/Utility.py
@@ -433,7 +433,7 @@ def do_sample() -> None:
     threading.Thread(target=do_sample).start()
 
 
-def simplify_filename(filename: str, replacement_char: str = '_', maximum_length: int = 64):
+def simplify_filename(filename: str, replacement_char: str = '_', maximum_length: int = 64) -> str:
     # This function replaces any illegal characters in a file name. Not a full path.
     # macOS illegal characters = \0 /
     # linux illegal characters = \0 /