diff --git a/src/nested_ragged_tensors/ragged_numpy.py b/src/nested_ragged_tensors/ragged_numpy.py
index 2d403dc..5805f18 100644
--- a/src/nested_ragged_tensors/ragged_numpy.py
+++ b/src/nested_ragged_tensors/ragged_numpy.py
@@ -825,6 +825,10 @@ def concatenate(cls, tensors: list) -> JointNestedRaggedTensorDict:
             tensors: The tensors to concatenate.
 
         Examples:
+            >>> JointNestedRaggedTensorDict.concatenate([])
+            Traceback (most recent call last):
+                ...
+            ValueError: Can't concatenate an empty list!
             >>> J1 = JointNestedRaggedTensorDict({
             ...     "T": [[1, 2, 3], [4, 5]],
             ...     "id": [[[1, 2, 3], [3, 4], [1, 2]], [[3], [3, 2, 2]]],
@@ -871,6 +875,30 @@ def concatenate(cls, tensors: list) -> JointNestedRaggedTensorDict:
                     [4. , 2. , 0. ],
                     [0. , 0. , 0. ],
                     [3. , 0. , 0. ]]])
+            >>> J1 = JointNestedRaggedTensorDict({
+            ...     "T": [1, 2, 3],
+            ...     "id": [[1, 2, 3], [3, 4], [1, 2]],
+            ... }, schema={"T": int, "id": int, "val": float})
+            >>> concatenated = JointNestedRaggedTensorDict.concatenate([J1])
+            >>> dense_dict = concatenated.to_dense()
+            >>> dense_dict['T']
+            array([1, 2, 3])
+            >>> J2 = JointNestedRaggedTensorDict({
+            ...     "T": [6, 7, 8, 9],
+            ...     "id": [[3], [3, 2, 2], [1], [1]],
+            ... }, schema={"T": int, "id": int, "val": float})
+            >>> concatenated = JointNestedRaggedTensorDict.concatenate([J1, J2])
+            >>> dense_dict = concatenated.to_dense()
+            >>> dense_dict['T']
+            array([1, 2, 3, 6, 7, 8, 9])
+            >>> dense_dict['id']
+            array([[1, 2, 3],
+                   [3, 4, 0],
+                   [1, 2, 0],
+                   [3, 0, 0],
+                   [3, 2, 2],
+                   [1, 0, 0],
+                   [1, 0, 0]])
         """
 
         if len(tensors) == 1:
@@ -912,10 +940,15 @@ def concatenate(cls, tensors: list) -> JointNestedRaggedTensorDict:
                         (out_tensors[bounds_key], T.tensors[bounds_key] + out_tensors[bounds_key][-1])
                     )
 
-                for key in out_keys_at_dim[dim]:
-                    out_tensors[f"dim{dim}/{key}"] = (
-                        out_tensors[f"dim{dim}/{key}"] + T.tensors[f"dim{dim}/{key}"]
-                    )
+                    for key in out_keys_at_dim[dim]:
+                        out_tensors[f"dim{dim}/{key}"] = (
+                            out_tensors[f"dim{dim}/{key}"] + T.tensors[f"dim{dim}/{key}"]
+                        )
+                elif dim == 0:
+                    for key in out_keys_at_dim[dim]:
+                        out_tensors[f"dim{dim}/{key}"] = np.concatenate(
+                            (out_tensors[f"dim{dim}/{key}"], T.tensors[f"dim{dim}/{key}"]), axis=0
+                        )
         return cls(out_tensors, pre_raggedified=True, schema=out_schema)
 
     @classmethod