diff --git a/nnunetv2/preprocessing/preprocessors/default_preprocessor.py b/nnunetv2/preprocessing/preprocessors/default_preprocessor.py
index df42ac993..7e0068b9d 100644
--- a/nnunetv2/preprocessing/preprocessors/default_preprocessor.py
+++ b/nnunetv2/preprocessing/preprocessors/default_preprocessor.py
@@ -14,7 +14,7 @@
 import multiprocessing
 import shutil
 from time import sleep
-from typing import Tuple
+from typing import Tuple, Union
 
 import numpy as np
 from batchgenerators.utilities.file_and_folder_operations import *