diff --git a/bob/bio/base/tools/preprocessor.py b/bob/bio/base/tools/preprocessor.py
index 95a3772a758462f3221ecd31cf58c98344c528be..86ec390c0f10f2adc4935841a31777341a6a019b 100644
--- a/bob/bio/base/tools/preprocessor.py
+++ b/bob/bio/base/tools/preprocessor.py
@@ -61,10 +61,7 @@ def preprocess(preprocessor, groups = None, indices = None, allow_missing_files
   for i in index_range:
     preprocessed_data_file = preprocessed_data_files[i]
     file_object = data_files[i]
-    if isinstance(file_object, list):
-      file_name = [f.make_path(original_directory, original_extension) for f in file_object]
-    else:
-      file_name = file_object.make_path(original_directory, original_extension)
+    file_name = file_object.make_path(original_directory, original_extension)
 
     # check for existence
     if not utils.check_file(preprocessed_data_file, force, 1000):