diff --git a/miplearn/features/sample.py b/miplearn/features/sample.py
index 41c7f52..078c847 100644
--- a/miplearn/features/sample.py
+++ b/miplearn/features/sample.py
@@ -207,7 +207,7 @@ class Hdf5Sample(Sample):
         if key not in self.file:
             return None
         ds = self.file[key]
-        lens = ds.attrs["lengths"]
+        lens = self.get_vector(f"{key}_lengths")
         if h5py.check_string_dtype(ds.dtype):
             padded = ds.asstr()[:].tolist()
         else:
@@ -238,6 +238,7 @@ class Hdf5Sample(Sample):
     def put_vector_list(self, key: str, value: VectorList) -> None:
         self._assert_is_vector_list(value)
         padded, lens = _pad(value)
+        self.put_vector(f"{key}_lengths", lens)
         data = None
         for v in value:
             if v is None or len(v) == 0:
@@ -251,8 +252,7 @@ class Hdf5Sample(Sample):
             break
         if data is None:
             data = np.array(padded)
-        ds = self._put(key, data)
-        ds.attrs["lengths"] = lens
+        self._put(key, data)
 
     def _put(self, key: str, value: Any) -> Dataset:
         if key in self.file: