diff --git a/src/pyrecest/filters/adaptive_process_noise.py b/src/pyrecest/filters/adaptive_process_noise.py
index fd74b3e98a..80cb89d652 100644
--- a/src/pyrecest/filters/adaptive_process_noise.py
+++ b/src/pyrecest/filters/adaptive_process_noise.py
@@ -194,15 +194,26 @@ def ratio(self, source_weights: Mapping[str, float] | None = None) -> float:
         if not self.ratios_by_source:
             return 1.0
         if source_weights:
-            numerator = 0.0
-            denominator = 0.0
+            weighted_ratios: list[tuple[float, float]] = []
             for source, ratio in self.ratios_by_source.items():
-                weight = float(source_weights.get(source, 0.0))
+                weight = _normalize_nonnegative_finite_scalar(
+                    source_weights.get(source, 0.0),
+                    f"source_weights[{source!r}]",
+                )
                 if weight > 0.0:
-                    numerator += weight * ratio
-                    denominator += weight
-            if denominator > 0.0:
-                return float(numerator / denominator)
+                    weighted_ratios.append((ratio, weight))
+            if weighted_ratios:
+                weight_scale = max(weight for _, weight in weighted_ratios)
+                aggregate = 0.0
+                total_weight = 0.0
+                for ratio, weight in weighted_ratios:
+                    scaled_weight = weight / weight_scale
+                    updated_total_weight = total_weight + scaled_weight
+                    aggregate += (scaled_weight / updated_total_weight) * (
+                        ratio - aggregate
+                    )
+                    total_weight = updated_total_weight
+                return float(aggregate)
         return float(np.mean(list(self.ratios_by_source.values())))
 
     def scale(self, source_weights: Mapping[str, float] | None = None) -> float:
diff --git a/tests/filters/test_adaptive_process_noise_source_weights.py b/tests/filters/test_adaptive_process_noise_source_weights.py
new file mode 100644
index 0000000000..21c93effcc
--- /dev/null
+++ b/tests/filters/test_adaptive_process_noise_source_weights.py
@@ -0,0 +1,51 @@
+"""Regression tests for weighted rolling NIS aggregation."""
+
+import numpy as np
+import pytest
+
+from pyrecest.filters.adaptive_process_noise import (
+    AdaptiveProcessNoiseConfig,
+    RollingNISProcessNoiseAdapter,
+)
+
+
+def _adapter_with_two_sources():
+    adapter = RollingNISProcessNoiseAdapter(
+        AdaptiveProcessNoiseConfig(ewma_alpha=1.0)
+    )
+    adapter.observe(source="radar", measurement_dim=1, nis=2.0)
+    adapter.observe(source="camera", measurement_dim=1, nis=4.0)
+    return adapter
+
+
+def test_weighted_ratio_remains_finite_for_maximum_finite_weights():
+    adapter = _adapter_with_two_sources()
+    weight = np.finfo(float).max
+
+    ratio = adapter.ratio({"radar": weight, "camera": weight})
+
+    assert np.isfinite(ratio)
+    assert ratio == pytest.approx(3.0)
+
+
+@pytest.mark.parametrize(
+    "weight",
+    [
+        np.nan,
+        np.inf,
+        -np.inf,
+        -1.0,
+        True,
+        "1.0",
+        np.array([1.0]),
+        np.timedelta64(1, "ns"),
+    ],
+)
+def test_weighted_ratio_rejects_invalid_source_weights(weight):
+    adapter = _adapter_with_two_sources()
+
+    with pytest.raises(
+        ValueError,
+        match=r"source_weights\['radar'\] must be a nonnegative finite scalar",
+    ):
+        adapter.ratio({"radar": weight, "camera": 1.0})