diff --git a/narwhals/_pandas_like/series.py b/narwhals/_pandas_like/series.py
index 33dad2ac6..39ffd4c2d 100644
--- a/narwhals/_pandas_like/series.py
+++ b/narwhals/_pandas_like/series.py
@@ -15,6 +15,7 @@
 if TYPE_CHECKING:
     from typing_extensions import Self
 
+    from narwhals._pandas_like.namespace import PandasNamespace
     from narwhals.dtypes import DType
 
 
@@ -45,6 +46,11 @@ def __init__(
         else:  # pragma: no cover
             pass
 
+    def __narwhals_namespace__(self) -> PandasNamespace:
+        from narwhals._pandas_like.namespace import PandasNamespace
+
+        return PandasNamespace(self._implementation)
+
     def __narwhals_series__(self) -> Self:
         return self
 
diff --git a/narwhals/series.py b/narwhals/series.py
index 31a622bc7..bb5898817 100644
--- a/narwhals/series.py
+++ b/narwhals/series.py
@@ -37,16 +37,10 @@ def __init__(
         raise TypeError(msg)  # pragma: no cover
 
     def _extract_native(self, arg: Any) -> Any:
-        from narwhals.expression import Expr
+        from narwhals.series import Series
 
-        if not self._is_polars:
-            return arg
         if isinstance(arg, Series):
             return arg._series
-        if isinstance(arg, Expr):
-            import polars as pl
-
-            return arg._call(pl)
         return arg
 
     def _from_series(self, series: Any) -> Self:
@@ -118,6 +112,9 @@ def n_unique(self) -> int:
     def unique(self) -> Self:
         return self._from_series(self._series.unique())
 
+    def sort(self) -> Self:
+        return self._from_series(self._series.sort())
+
     def zip_with(self, mask: Self, other: Self) -> Self:
         return self._from_series(
             self._series.zip_with(self._extract_native(mask), self._extract_native(other))
@@ -147,7 +144,7 @@ def std(self) -> Any:
         return self._series.std()
 
     def __gt__(self, other: Any) -> Series:
-        return self._series.__gt__(self._extract_native(other))  # type: ignore[no-any-return]
+        return self._from_series(self._series.__gt__(self._extract_native(other)))
 
     def __lt__(self, other: Any) -> Series:
-        return self._series.__lt__(self._extract_native(other))  # type: ignore[no-any-return]
+        return self._from_series(self._series.__lt__(self._extract_native(other)))
diff --git a/tests/test_common.py b/tests/test_common.py
index 53922191e..a570e5303 100644
--- a/tests/test_common.py
+++ b/tests/test_common.py
@@ -496,3 +496,16 @@ def test_any_all(df_raw: Any) -> None:
     result = nw.to_native(df.select((nw.all() > 1).any()))
     expected = {"a": [True], "b": [True], "z": [True]}
     compare_dicts(result, expected)
+
+
+def test_invalid() -> None:
+    df = nw.LazyFrame(df_pandas)
+    with pytest.raises(ValueError, match="Multi-output"):
+        df.select(nw.all() + nw.all())
+
+
+@pytest.mark.parametrize("df_raw", [df_pandas])
+def test_reindex(df_raw: Any) -> None:
+    df = nw.DataFrame(df_raw)
+    with pytest.raises(RuntimeError, match="implicit index alignment"):
+        df.select("a", df["b"].sort())
diff --git a/tests/test_series.py b/tests/test_series.py
index 9f4d1c3f9..7ced67eed 100644
--- a/tests/test_series.py
+++ b/tests/test_series.py
@@ -29,6 +29,19 @@ def test_is_in(df_raw: Any) -> None:
     assert result[2]
 
 
+@pytest.mark.parametrize("df_raw", [df_pandas, df_polars])
+def test_gt(df_raw: Any) -> None:
+    s = nw.Series(df_raw["a"])
+    result = nw.to_native(s > s)  # noqa: PLR0124
+    assert not result[0]
+    assert not result[1]
+    assert not result[2]
+    result = nw.to_native(s > 1)
+    assert not result[0]
+    assert result[1]
+    assert result[2]
+
+
 @pytest.mark.parametrize("df_raw", [df_pandas, df_lazy])
 def test_dtype(df_raw: Any) -> None:
     result = nw.LazyFrame(df_raw).collect()["a"].dtype