diff --git a/python/rateslib/periods/protocols/fixings.py b/python/rateslib/periods/protocols/fixings.py
index 966eec22..10f043cc 100644
--- a/python/rateslib/periods/protocols/fixings.py
+++ b/python/rateslib/periods/protocols/fixings.py
@@ -227,7 +227,7 @@ def _replace_fixings_with_ad_variables(
                 for d, v in identifier[1].items()
             ],
         )
-        index = index.union(other=ad_series.index, sort=True)  # type: ignore[arg-type]
+        index = index.union(other=ad_series.index, sort=None)  # type: ignore[arg-type]  # will sort
         fixings.pop(name=identifier[0])
         fixings.add(
             name=identifier[0],
diff --git a/python/tests/test_curves.py b/python/tests/test_curves.py
index bcb487d9..08f72612 100644
--- a/python/tests/test_curves.py
+++ b/python/tests/test_curves.py
@@ -2668,7 +2668,7 @@ def test_curve_method_from_curve_with_non_zero_index_lag(self):
         ],
     )
     def test_series_len_zero(self, curve, exp):
-        s = Series(data=[], index=[])
+        s = Series(data=[], index=[], dtype=float)
         result = _try_index_value(0, "curve", s, dt(2000, 1, 1), curve)
         assert isinstance(result, exp)