diff --git a/docs/spelling_wordlist.txt b/docs/spelling_wordlist.txt
index 7eaf69004..1a831d75c 100644
--- a/docs/spelling_wordlist.txt
+++ b/docs/spelling_wordlist.txt
@@ -166,3 +166,13 @@ isort
 yapf
 pydocstyle
 Args
+tuples
+tuple
+Multi
+multi
+parameterized
+Proximal
+metadata
+GPU
+Dopamine
+builtin
diff --git a/test/base/test_buffer.py b/test/base/test_buffer.py
index 203d8e4fb..3ffdeccb9 100644
--- a/test/base/test_buffer.py
+++ b/test/base/test_buffer.py
@@ -1336,6 +1336,65 @@ def test_from_data():
     os.remove(path)
 
 
+def test_custom_key():
+    batch = Batch(
+        **{
+            'obs_next':
+            np.array(
+                [
+                    [
+                        1.174, -0.1151, -0.609, -0.5205, -0.9316, 3.236, -2.418, 0.386,
+                        0.2227, -0.5117, 2.293
+                    ]
+                ]
+            ),
+            'rew':
+            np.array([4.28125]),
+            'act':
+            np.array([[-0.3088, -0.4636, 0.4956]]),
+            'truncated':
+            np.array([False]),
+            'obs':
+            np.array(
+                [
+                    [
+                        1.193, -0.1203, -0.6123, -0.519, -0.9434, 3.32, -2.266, 0.9116,
+                        0.623, 0.1259, 0.363
+                    ]
+                ]
+            ),
+            'terminated':
+            np.array([False]),
+            'done':
+            np.array([False]),
+            'returns':
+            np.array([74.70343082]),
+            'info':
+            Batch(),
+            'policy':
+            Batch(),
+        }
+    )
+    buffer_size = len(batch.rew)
+    buffer = ReplayBuffer(buffer_size)
+    buffer.add(batch)
+    sampled_batch, _ = buffer.sample(1)
+    # Check if they have the same keys
+    assert set(batch.keys()) == set(sampled_batch.keys()), \
+        "Batches have different keys: {} and {}".format(
+            set(batch.keys()), set(sampled_batch.keys()))
+    # Compare the values for each key
+    for key in batch.keys():
+        if isinstance(batch.__dict__[key], np.ndarray
+                      ) and isinstance(sampled_batch.__dict__[key], np.ndarray):
+            assert np.allclose(batch.__dict__[key], sampled_batch.__dict__[key]), \
+                "Value mismatch for key: {}".format(key)
+        if isinstance(batch.__dict__[key],
+                      Batch) and isinstance(sampled_batch.__dict__[key], Batch):
+            assert batch.__dict__[key].is_empty()
+            assert sampled_batch.__dict__[key].is_empty()
+
+
 if __name__ == '__main__':
     test_replaybuffer()
     test_ignore_obs_next()
@@ -1351,3 +1410,4 @@ def test_from_data():
     test_multibuf_hdf5()
     test_from_data()
     test_herreplaybuffer()
+    test_custom_key()
diff --git a/tianshou/data/buffer/base.py b/tianshou/data/buffer/base.py
index aa159bd34..6294ec36d 100644
--- a/tianshou/data/buffer/base.py
+++ b/tianshou/data/buffer/base.py
@@ -220,9 +220,8 @@ def add(
     ) -> Tuple[np.ndarray, np.ndarray, np.ndarray, np.ndarray]:
         """Add a batch of data into replay buffer.
 
-        :param Batch batch: the input data batch. Its keys must belong to the 7
-            input keys, and "obs", "act", "rew", "terminated", "truncated" is
-            required.
+        :param Batch batch: the input data batch. "obs", "act", "rew",
+            "terminated", "truncated" are required keys.
         :param buffer_ids: to make consistent with other buffer's add function; if it
             is not None, we assume the input batch's first dimension is always 1.
 
@@ -232,12 +231,12 @@ def add(
         """
         # preprocess batch
         new_batch = Batch()
-        for key in set(self._input_keys).intersection(batch.keys()):
+        for key in batch.keys():
             new_batch.__dict__[key] = batch[key]
         batch = new_batch
         batch.__dict__["done"] = np.logical_or(batch.terminated, batch.truncated)
-        assert set(["obs", "act", "rew", "terminated", "truncated",
-                    "done"]).issubset(batch.keys())
+        assert set(["obs", "act", "rew", "terminated", "truncated", "done"]
+                   ).issubset(batch.keys())  # important to do after preprocess batch
         stacked_batch = buffer_ids is not None
         if stacked_batch:
             assert len(batch) == 1
@@ -376,14 +375,18 @@ def __getitem__(self, index: Union[slice, int, List[int], np.ndarray]) -> Batch:
             obs_next = self.get(indices, "obs_next", Batch())
         else:
             obs_next = self.get(self.next(indices), "obs", Batch())
-        return Batch(
-            obs=obs,
-            act=self.act[indices],
-            rew=self.rew[indices],
-            terminated=self.terminated[indices],
-            truncated=self.truncated[indices],
-            done=self.done[indices],
-            obs_next=obs_next,
-            info=self.get(indices, "info", Batch()),
-            policy=self.get(indices, "policy", Batch()),
-        )
+        batch_dict = {
+            "obs": obs,
+            "act": self.act[indices],
+            "rew": self.rew[indices],
+            "terminated": self.terminated[indices],
+            "truncated": self.truncated[indices],
+            "done": self.done[indices],
+            "obs_next": obs_next,
+            "info": self.get(indices, "info", Batch()),
+            "policy": self.get(indices, "policy", Batch()),
+        }
+        for key in self._meta.__dict__.keys():
+            if key not in self._input_keys:
+                batch_dict[key] = self._meta[key][indices]
+        return Batch(batch_dict)