diff --git a/crates/engine/src/schema/plastic.rs b/crates/engine/src/schema/plastic.rs
index 24d16a2f47..232c4473b2 100644
--- a/crates/engine/src/schema/plastic.rs
+++ b/crates/engine/src/schema/plastic.rs
@@ -117,15 +117,27 @@ fn rule_name_schema(_: &mut SchemaGenerator) -> Schema {
 #[schemars(deny_unknown_fields, transform = bodies_required)]
 pub struct AssignRuleParams {
     /// The bodies, by name: 1 to 10 000.
-    // Optional in the type, required in the schema: left out or empty, the handler refuses the
-    // call with its own message.
+    // Optional in the type, required in the schema: left out, `null` or empty, the handler
+    // refuses the call with its own message, so the schema allows none of those.
     #[serde(default)]
+    #[schemars(schema_with = "bodies_schema")]
     pub bodies: Option<NameList>,
     /// The rule, by name, as `plastic.rules` lists it. Empty or left out: the bodies have no
     /// rule of their own, so the active rule applies.
     pub rule: Option<String>,
 }
 
+/// `bodies` as `plastic.assign_rule` takes it: a name, or a list of 1 to 10 000 names. Not
+/// `NameList`'s own schema, which allows the empty list the handler refuses.
+fn bodies_schema(_: &mut SchemaGenerator) -> Schema {
+    json_schema!({
+        "anyOf": [
+            {"type": "string"},
+            {"type": "array", "items": {"type": "string"}, "minItems": 1, "maxItems": crate::params::MAX_STRING_LIST}
+        ]
+    })
+}
+
 /// List the plastic rules (the library's and the design's), the active rule and the rules
 /// assigned to bodies. No parameters.
 #[derive(Debug, Deserialize, JsonSchema)]
diff --git a/crates/engine/src/schema/plastic_tests.rs b/crates/engine/src/schema/plastic_tests.rs
index 9ba62e2227..d4e14f49b2 100644
--- a/crates/engine/src/schema/plastic_tests.rs
+++ b/crates/engine/src/schema/plastic_tests.rs
@@ -93,7 +93,7 @@ fn the_contracts_refuse_what_the_handlers_ignore() {
 
 /// #12313: the schemas take what the handlers read: a rule's name as text, its values as
 /// numbers or expressions, `active` and `delete` as true or false; bodies as a name or a list of
-/// names, and the rule as text or `null`.
+/// one or more names (not `null`), and the rule as text or `null`.
 #[test]
 fn the_schemas_take_what_the_handlers_read() {
     let manage = input_schema_for::<ManageRulesParams>();
@@ -119,6 +119,8 @@ fn the_schemas_take_what_the_handlers_read() {
         (json!({"bodies": ["Body1"], "rule": null}), true),
         (json!({}), false),
         (json!({"bodies": [5]}), false),
+        (json!({"bodies": null}), false),
+        (json!({"bodies": []}), false),
         (json!({"bodies": ["Body1"], "rule": 5}), false),
     ] {
         assert_eq!(conforms(assign, assign, &p), takes, "{p}");