diff --git a/src/agentrust_telemetry/adapters/opa.py b/src/agentrust_telemetry/adapters/opa.py
index 065e239..a4a3194 100644
--- a/src/agentrust_telemetry/adapters/opa.py
+++ b/src/agentrust_telemetry/adapters/opa.py
@@ -35,7 +35,13 @@ def opa_decision_log(
         raise ValueError("OPA decision log requires a non-empty decision_id")
     mapper = result_mapper or _boolean_decision
     decision = mapper(decision_log.get("result"))
-    if decision not in {"allow", "deny", "challenge", "not_applicable", "error"}:
+    if not isinstance(decision, str) or decision not in {
+        "allow",
+        "deny",
+        "challenge",
+        "not_applicable",
+        "error",
+    }:
         raise ValueError(f"OPA result mapper returned unsupported decision: {decision!r}")
     labels = decision_log.get("labels", {})
     if not isinstance(labels, dict):
diff --git a/tests/test_adapters.py b/tests/test_adapters.py
index d67cd93..14b879d 100644
--- a/tests/test_adapters.py
+++ b/tests/test_adapters.py
@@ -175,11 +175,17 @@ def test_opa_requires_non_empty_string_decision_id(self):
                                  "OPA decision log requires a non-empty decision_id")
 
     def test_opa_rejects_unsupported_mapper_decision(self):
-        with self.assertRaises(ValueError) as caught:
-            self.opa_event({"decision_id": "d-1", "result": {}},
-                           result_mapper=lambda value: "permit")
-        self.assertEqual(str(caught.exception),
-                         "OPA result mapper returned unsupported decision: 'permit'")
+        for decision in ("permit", {}, []):
+            with self.subTest(decision=decision):
+                with self.assertRaises(ValueError) as caught:
+                    self.opa_event(
+                        {"decision_id": "d-1", "result": {}},
+                        result_mapper=lambda value, decision=decision: decision,
+                    )
+                self.assertEqual(
+                    str(caught.exception),
+                    f"OPA result mapper returned unsupported decision: {decision!r}",
+                )
 
     def test_opa_requires_object_labels(self):
         for labels in ([], None, "version=1.8.0"):