diff --git a/crates/engine/src/cmd/slide.rs b/crates/engine/src/cmd/slide.rs
index 777c479c..88ee9685 100644
--- a/crates/engine/src/cmd/slide.rs
+++ b/crates/engine/src/cmd/slide.rs
@@ -373,7 +373,7 @@ fn select_slides(s: &mut Session, p: &Value) -> Result<Value> {
 }
 
 fn hide(s: &mut Session, p: &Value) -> Result<Value> {
-    let targets: Vec<usize> = match usize_param(p, "index") {
+    let targets: Vec<usize> = match opt_usize_param(p, "index", "slide.hide")? {
         Some(i) => vec![i],
         None => {
             let st = s.doc()?;
@@ -442,7 +442,7 @@ pub fn apply_layout(doc: &mut Presentation, index: usize, layout: LayoutId) -> R
 
 fn set_layout(s: &mut Session, p: &Value) -> Result<Value> {
     let key = p.get("layout").cloned().ok_or_else(|| bad("slide.layout", "missing `layout`"))?;
-    let targets: Vec<usize> = match usize_param(p, "index") {
+    let targets: Vec<usize> = match opt_usize_param(p, "index", "slide.layout")? {
         Some(i) => vec![i],
         None => {
             let st = s.doc()?;
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index 35b0ab39..2c1ed0be 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -4403,3 +4403,18 @@ fn duplicating_a_moved_copy_repeats_the_move() {
     assert_eq!(boxed(&mut s, f), (340.0 + off, 100.0 + off));
     assert_eq!(s.doc().unwrap().doc.validate(), Vec::<String>::new());
 }
+
+#[test]
+fn slide_hide_and_layout_reject_invalid_index() {
+    let mut s = session();
+    s.execute("slide.new", &json!({})).unwrap();
+    s.execute("slide.new", &json!({})).unwrap();
+    s.execute("slide.selectSlides", &json!({"indices": [0]})).unwrap();
+    for bad in [json!(-1), json!(1.5)] {
+        let e = s.execute("slide.hide", &json!({"index": bad})).unwrap_err().to_string();
+        assert!(e.contains("not a valid index"), "{e}");
+        let e = s.execute("slide.layout", &json!({"layout": "blank", "index": bad})).unwrap_err().to_string();
+        assert!(e.contains("not a valid index"), "{e}");
+    }
+    assert!(s.doc().unwrap().doc.slides.iter().all(|x| !x.hidden));
+}