diff --git a/miplearn/features/sample.py b/miplearn/features/sample.py
index 11f37b4..4f33edd 100644
--- a/miplearn/features/sample.py
+++ b/miplearn/features/sample.py
@@ -312,6 +312,7 @@ class Hdf5Sample(Sample):
 
 
 def _pad(veclist: VectorList) -> Tuple[VectorList, List[int]]:
+    veclist = deepcopy(veclist)
     lens = [len(v) if v is not None else -1 for v in veclist]
     maxlen = max(lens)
 
diff --git a/tests/features/test_extractor.py b/tests/features/test_extractor.py
index 9edb496..9612211 100644
--- a/tests/features/test_extractor.py
+++ b/tests/features/test_extractor.py
@@ -192,7 +192,7 @@ if __name__ == "__main__":
     extractor = FeaturesExtractor(with_lhs=False)
     sample = Hdf5Sample("tmp/prof.h5", mode="w")
 
-    def run():
+    def run() -> None:
         extractor.extract_after_load_features(instance, solver, sample)
         extractor.extract_after_lp_features(solver, sample)