diff --git a/src/doxa/opinion.py b/src/doxa/opinion.py
index baa9471..d76bb15 100644
--- a/src/doxa/opinion.py
+++ b/src/doxa/opinion.py
@@ -26,6 +26,13 @@
 _BASE_RATE_CLAMP = (0.01, 0.99)
 
 
+def _require_finite(**coordinates: float) -> None:
+    """Reject coordinates outside the finite real numbers."""
+    for name, value in coordinates.items():
+        if not math.isfinite(value):
+            raise ValueError(f"{name}={value} must be finite")
+
+
 def _clamp_base_rate(a: float) -> float:
     """Clamp a fused base rate to ``_BASE_RATE_CLAMP``."""
     lo, hi = _BASE_RATE_CLAMP
@@ -47,6 +54,7 @@ class Opinion:
     allow_dogmatic: bool = False
 
     def __post_init__(self) -> None:
+        _require_finite(b=self.b, d=self.d, u=self.u, a=self.a)
         for name, val in [("b", self.b), ("d", self.d), ("u", self.u)]:
             if val < -_TOL or val > 1.0 + _TOL:
                 raise ValueError(f"{name}={val} not in [0, 1]")
@@ -575,6 +583,7 @@ class BetaEvidence:
     a: float
 
     def __post_init__(self) -> None:
+        _require_finite(r=self.r, s=self.s, a=self.a)
         if self.r < 0:
             raise ValueError(f"r={self.r} must be >= 0")
         if self.s < 0:
diff --git a/tests/test_opinion.py b/tests/test_opinion.py
index 16a4418..da16609 100644
--- a/tests/test_opinion.py
+++ b/tests/test_opinion.py
@@ -95,6 +95,32 @@ def test_a_valid(self):
         assert 0.0 < o.a < 1.0
 
 
+class TestFiniteCoordinates:
+    @pytest.mark.parametrize("value", [math.nan, math.inf, -math.inf])
+    @pytest.mark.parametrize("coordinate", ["b", "d", "u", "a"])
+    def test_opinion_rejects_non_finite_coordinate(self, coordinate, value):
+        coordinates = {"b": 0.0, "d": 0.0, "u": 1.0, "a": 0.5}
+        coordinates[coordinate] = value
+
+        with pytest.raises(
+            ValueError,
+            match=rf"^{coordinate}=.* must be finite$",
+        ):
+            Opinion(**coordinates)
+
+    @pytest.mark.parametrize("value", [math.nan, math.inf, -math.inf])
+    @pytest.mark.parametrize("coordinate", ["r", "s", "a"])
+    def test_beta_evidence_rejects_non_finite_coordinate(self, coordinate, value):
+        coordinates = {"r": 0.0, "s": 0.0, "a": 0.5}
+        coordinates[coordinate] = value
+
+        with pytest.raises(
+            ValueError,
+            match=rf"^{coordinate}=.* must be finite$",
+        ):
+            BetaEvidence(**coordinates)
+
+
 # --- 3. E(ω) = b + a*u is in [0, 1] ---
 
 class TestExpectation: