diff --git a/energypylinear/assets/site.py b/energypylinear/assets/site.py
index 0c19edf..a578c2f 100644
--- a/energypylinear/assets/site.py
+++ b/energypylinear/assets/site.py
@@ -27,7 +27,7 @@ def get_custom_interval_data(kwargs: dict | None) -> list | None:
     Returns:
         A list of custom interval data, or `None` if no kwargs.
     """
-    if kwargs is None:
+    if (kwargs is None) or (kwargs == {}):
         return None
 
     custom_interval_data = []
diff --git a/tests/assets/test_battery.py b/tests/assets/test_battery.py
index 9ec0683..2274d8c 100644
--- a/tests/assets/test_battery.py
+++ b/tests/assets/test_battery.py
@@ -48,6 +48,20 @@ def test_price_optimization(
     dispatch = charge - discharge
     np.testing.assert_almost_equal(dispatch, expected_dispatch)
 
+    # now try the same with a spill asset
+    # this is just for test coverage really...
+    asset = epl.Battery(
+        power_mw=power_mw,
+        capacity_mwh=capacity_mwh,
+        efficiency_pct=efficiency,
+        electricity_prices=np.array(electricity_prices),
+        freq_mins=freq_mins,
+        initial_charge_mwh=initial_charge_mwh,
+        final_charge_mwh=0,
+        include_spill=True,
+    )
+    simulation = asset.optimize(verbose=False)
+
 
 @pytest.mark.parametrize(
     "carbon_intensities, initial_charge_mwh, expected_dispatch",
@@ -322,7 +336,6 @@ def test_no_simultaneous_import_export() -> None:
     )
     simulation = asset.optimize()
     results = simulation.results
-
     check_no_simultaneous(results, "site-import_power_mwh", "site-export_power_mwh")
 
 
diff --git a/tests/assets/test_renewable_generator.py b/tests/assets/test_renewable_generator.py
index 0cbd49e..568a7d9 100644
--- a/tests/assets/test_renewable_generator.py
+++ b/tests/assets/test_renewable_generator.py
@@ -123,6 +123,7 @@ def test_interval_data() -> None:
     electric_generation_lower_bound_pct=hypothesis.strategies.floats(
         min_value=0, max_value=1.0
     ),
+    include_spill=hypothesis.strategies.booleans(),
 )
 def test_hypothesis(
     idx_length: int,
@@ -130,6 +131,7 @@ def test_hypothesis(
     prices_std: float,
     prices_offset: float,
     electric_generation_lower_bound_pct: float,
+    include_spill: bool,
 ) -> None:
     """Test optimization with hypothesis."""
     electricity_prices = (
@@ -144,5 +146,6 @@ def test_hypothesis(
         electricity_prices=electricity_prices,
         electric_generation_mwh=electric_generation_mwh,
         electric_generation_lower_bound_pct=electric_generation_lower_bound_pct,
+        include_spill=include_spill,
     )
     asset.optimize(verbose=False)