diff --git a/crates/engine/src/cmd/component.rs b/crates/engine/src/cmd/component.rs
index ff7a56df..42730a37 100644
--- a/crates/engine/src/cmd/component.rs
+++ b/crates/engine/src/cmd/component.rs
@@ -608,7 +608,7 @@ fn make_slot(s: &mut Session, p: &Value) -> Result<Value> {
     let owner = Owner::resolve(doc, id).ok_or_else(|| bad(cmd, "the frame isn't inside a component"))?;
     let name = string(p, "name").unwrap_or_else(|| fresh_prop_name(doc, &owner, &n.name));
     let preferred = get(p, "preferred").cloned().unwrap_or(Value::Null);
-    add_prop(s, &json!({"id": id, "layer": id, "name": name, "type": "slot", "preferred": preferred}))
+    add_prop_for(cmd, s, &json!({"id": id, "layer": id, "name": name, "type": "slot", "preferred": preferred}))
 }
 
 fn remove_slot(s: &mut Session, p: &Value) -> Result<Value> {
@@ -1039,7 +1039,12 @@ fn valid_prop_name(cmd: &str, name: &str) -> Result<String> {
 }
 
 fn add_prop(s: &mut Session, p: &Value) -> Result<Value> {
-    let cmd = "component.addProperty";
+    add_prop_for("component.addProperty", s, p)
+}
+
+/// Add a component property for command `cmd`, which errors name (`component.makeSlot` adds its
+/// slot property here).
+fn add_prop_for(cmd: &str, s: &mut Session, p: &Value) -> Result<Value> {
     let target = targets(s, p).first().copied().ok_or_else(|| bad(cmd, "select a component"))?;
     let name = valid_prop_name(cmd, &req_string(cmd, p, "name")?)?;
     let t = req_string(cmd, p, "type")?;
diff --git a/crates/engine/src/slot_tests.rs b/crates/engine/src/slot_tests.rs
index 516cb9d6..cbf824d9 100644
--- a/crates/engine/src/slot_tests.rs
+++ b/crates/engine/src/slot_tests.rs
@@ -41,6 +41,17 @@ fn props_of(s: &mut Session, id: NodeId) -> Vec<Value> {
     ex(s, "component.properties", json!({"id": id}))["properties"].as_array().cloned().unwrap_or_default()
 }
 
+/// A refused slot is reported by `component.makeSlot`, the command that was run, not by the
+/// property command it adds its slot property through.
+#[test]
+fn a_refused_slot_names_make_slot() {
+    let mut s = Session::new();
+    let (c, ..) = slot_card(&mut s);
+    let other = idv(&ex(&mut s, "frame.create", json!({"x": 150, "y": 10, "width": 40, "height": 30, "parent": c, "name": "Other"})));
+    let err = s.execute("component.makeSlot", &json!({"id": other, "name": "Content"})).unwrap_err().to_string();
+    assert!(err.contains("`component.makeSlot`") && err.contains("a property named `Content` already exists"), "{err}");
+}
+
 #[test]
 fn slots_are_frames_of_a_main_with_a_slot_property() {
     let mut s = Session::new();