diff --git a/crates/athena-sim/tests/scenario_schema.rs b/crates/athena-sim/tests/scenario_schema.rs
index d51d234..62357ef 100644
--- a/crates/athena-sim/tests/scenario_schema.rs
+++ b/crates/athena-sim/tests/scenario_schema.rs
@@ -4,6 +4,9 @@
 use std::fs;
 use std::path::PathBuf;
 
+use athena_sim::{RunError, run_scenario};
+use serde_json::{Value, json};
+
 fn docs() -> PathBuf {
     PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("../../docs")
 }
@@ -39,3 +42,146 @@ fn schema_accepts_valid_and_rejects_invalid_examples() {
     }
     assert!(invalid >= 3, "expected at least three invalid examples");
 }
+
+/// The schema fragments of one object, with `$ref` followed.
+fn resolve<'a>(root: &'a Value, schema: &'a Value) -> &'a Value {
+    match schema["$ref"].as_str() {
+        Some(reference) => resolve(root, root.pointer(&reference[1..]).unwrap()),
+        None => schema,
+    }
+}
+
+/// The first value of `key` among the fragments.
+fn first<'a>(fragments: &[&'a Value], key: &str) -> Option<&'a Value> {
+    fragments.iter().find_map(|fragment| fragment.get(key))
+}
+
+/// Collect the paths of optional properties the schema types as an object or
+/// an array. Array items are entered at index 0. The `if`/`then`/`else`
+/// branches of `allOf` apply for `version`.
+fn optional_structured(
+    root: &Value,
+    fragments: &[&Value],
+    version: u64,
+    path: &mut Vec<Value>,
+    out: &mut Vec<Vec<Value>>,
+) {
+    let mut fragments = fragments.to_vec();
+    for entry in fragments
+        .iter()
+        .flat_map(|f| f["allOf"].as_array().into_iter().flatten())
+        .collect::<Vec<_>>()
+    {
+        let matches = entry["if"]["properties"]["scenarioVersion"]["const"] == version;
+        let branch = if matches {
+            &entry["then"]
+        } else {
+            &entry["else"]
+        };
+        fragments.push(branch);
+    }
+    let required: Vec<&str> = fragments
+        .iter()
+        .flat_map(|f| f["required"].as_array().into_iter().flatten())
+        .filter_map(Value::as_str)
+        .collect();
+    let mut names: Vec<&String> = fragments
+        .iter()
+        .flat_map(|f| {
+            f["properties"]
+                .as_object()
+                .into_iter()
+                .flat_map(|p| p.keys())
+        })
+        .collect();
+    names.sort();
+    names.dedup();
+    for name in names {
+        let property: Vec<&Value> = fragments
+            .iter()
+            .filter_map(|f| f["properties"].get(name))
+            .map(|schema| resolve(root, schema))
+            .collect();
+        let kind = first(&property, "type").and_then(Value::as_str);
+        path.push(json!(name));
+        match kind {
+            Some("object") => {
+                if !required.contains(&name.as_str()) {
+                    out.push(path.clone());
+                }
+                optional_structured(root, &property, version, path, out);
+            }
+            Some("array") => {
+                if !required.contains(&name.as_str()) {
+                    out.push(path.clone());
+                }
+                if let Some(items) = first(&property, "items") {
+                    path.push(json!(0));
+                    optional_structured(root, &[resolve(root, items)], version, path, out);
+                    path.pop();
+                }
+            }
+            _ => {}
+        }
+        path.pop();
+    }
+}
+
+/// Set the value at `path`, creating missing objects. An array on the path
+/// must already hold its first element.
+fn set_at(doc: &mut Value, path: &[Value], value: Value) {
+    let mut node = doc;
+    for (i, segment) in path.iter().enumerate() {
+        let last = i + 1 == path.len();
+        node = match segment {
+            Value::String(key) if last => {
+                node[key.as_str()] = value;
+                return;
+            }
+            Value::String(key) => node
+                .as_object_mut()
+                .unwrap()
+                .entry(key.as_str())
+                .or_insert_with(|| json!({})),
+            index => &mut node[index.as_u64().unwrap() as usize],
+        };
+    }
+}
+
+/// An explicit `null` on an optional object or array field is not the same
+/// as leaving the field out: the loader refuses it (athena#126, #141). The
+/// fields come from the schema, so a new one is covered without an edit.
+#[test]
+fn explicit_null_on_an_optional_structured_field_is_refused() {
+    let schema = read(docs().join("scenario.schema.json"));
+    let program = "rule (\"r\") { event { Ongoing - Global; } }";
+    let mut checked = 0;
+    for version in [1, 2] {
+        // Version 2 renames the player variables from `state` to `vars`.
+        let mut base = read(docs().join("scenario-examples/valid/practice-range.json"));
+        if version == 2 {
+            base["scenarioVersion"] = json!(2);
+            let player = base["players"][0].as_object_mut().unwrap();
+            let vars = player.remove("state").unwrap();
+            player.insert("vars".into(), vars);
+        }
+        assert!(
+            run_scenario(program, &base.to_string()).is_ok(),
+            "version {version} base does not load"
+        );
+        let mut fields = Vec::new();
+        optional_structured(&schema, &[&schema], version, &mut Vec::new(), &mut fields);
+        for path in fields {
+            let mut doc = base.clone();
+            set_at(&mut doc, &path, Value::Null);
+            match run_scenario(program, &doc.to_string()) {
+                Err(RunError::Scenario(_)) => checked += 1,
+                other => panic!(
+                    "version {version}: {} set to null: {other:?}",
+                    Value::Array(path)
+                ),
+            }
+        }
+    }
+    assert!(checked >= 10, "the schema walk found only {checked} fields");
+}