diff --git a/decider/systemone.py b/decider/systemone.py
index 02b1d18..9950e1d 100644
--- a/decider/systemone.py
+++ b/decider/systemone.py
@@ -54,7 +54,9 @@ def render_question(spec):
             raise ValueError(f"score criteria: an ordered list of 2..{MAX_LEVELS} level descriptions")
         names = list(range(len(crit))); opts = [f"{i}: {_txt(c)}" for i, c in enumerate(crit)]
     elif t in ("noul", "bool"):
-        names = [False, True]; c = crit or {}
+        if crit is not None and not isinstance(crit, dict):
+            raise ValueError("noul criteria: a map of optional true/false descriptions")
+        names = [False, True]; c = crit if crit is not None else {}
         f, tr = c.get("false", c.get(False)), c.get("true", c.get(True))
         opts = ["no" if f in (None, "") else f"no: {_txt(f)}", "yes" if tr in (None, "") else f"yes: {_txt(tr)}"]
     else:
diff --git a/tests/test_serve_http.py b/tests/test_serve_http.py
index f9ab991..a328f30 100644
--- a/tests/test_serve_http.py
+++ b/tests/test_serve_http.py
@@ -142,6 +142,20 @@ async def fn(cl):
     assert r.status_code == 422 and r.json() == {"detail": "choice criteria: a map of 2..255 options"}
 
 
+@pytest.mark.parametrize("question_type", ["noul", "bool"])
+@pytest.mark.parametrize("criteria", [["bad"], []])
+def test_invalid_noul_criteria_is_422_with_detail(served, question_type, criteria):
+    eng, run = served
+
+    async def fn(cl):
+        return await cl.post("/v1/systemone", json={"state": "s", "questions": {"q": {
+            "type": question_type, "instructions": "Is this true?", "criteria": criteria}}})
+    r = run(fn)
+    assert r.status_code == 422
+    assert r.json() == {"detail": "noul criteria: a map of optional true/false descriptions"}
+    assert eng.stats["forwards"] == 0
+
+
 def test_shared_path_is_used_for_long_multi_question_states(served, monkeypatch):
     eng, run = served
     monkeypatch.setattr(serve, "SHARED", True); monkeypatch.setattr(serve, "SHARED_MIN_TOKENS", 100)
diff --git a/tests/test_systemone.py b/tests/test_systemone.py
index 842c18f..869e0c4 100644
--- a/tests/test_systemone.py
+++ b/tests/test_systemone.py
@@ -36,6 +36,13 @@ def test_noul_with_and_without_criteria():
     assert desc["options"] == ["no: anything else", "yes: money back"]
 
 
+@pytest.mark.parametrize("question_type", ["noul", "bool"])
+@pytest.mark.parametrize("criteria", [["bad"], [], "bad", False])
+def test_noul_criteria_must_be_a_map(question_type, criteria):
+    with pytest.raises(ValueError, match="noul criteria: a map of optional true/false descriptions"):
+        s1.render_question({"type": question_type, "instructions": "Refund asked?", "criteria": criteria})
+
+
 def test_render_state_serialises_json_and_indexes_long_arrays():
     assert s1.render_state("plain text") == "plain text"
     short = json.loads(s1.render_state({"items": [{"a": 1}, {"a": 2}]}))