diff --git a/crates/engine/src/cmd/design/slide_background.rs b/crates/engine/src/cmd/design/slide_background.rs
index 0de78e7e..92f3ce77 100644
--- a/crates/engine/src/cmd/design/slide_background.rs
+++ b/crates/engine/src/cmd/design/slide_background.rs
@@ -1,6 +1,6 @@
 use super::{
     Arc, BTreeSet, Background, Fill, NO_ID_SPACE, Presentation, Result, Session, Target, Value, bad, bool_or, bool_param, color_param, color_value,
-    f64_param, ok, usize_param, with_param,
+    f64_param, ok, opt_usize_param, usize_param, with_param,
 };
 
 pub(super) fn background(s: &mut Session, p: &Value) -> Result<Value> {
@@ -62,7 +62,7 @@ pub(super) fn background(s: &mut Session, p: &Value) -> Result<Value> {
         ));
     };
     let all = bool_or(p, "all", false);
-    let index = usize_param(p, "index");
+    let index = opt_usize_param(p, "index", "design.background")?;
     if matches!(op, BgOp::Adjust) {
         let next = {
             let st = s.doc()?;
diff --git a/crates/engine/src/cmd/design/tests.rs b/crates/engine/src/cmd/design/tests.rs
index 9765fcc8..82eed6fe 100644
--- a/crates/engine/src/cmd/design/tests.rs
+++ b/crates/engine/src/cmd/design/tests.rs
@@ -485,3 +485,16 @@ fn background_all_clears_layout_overrides_even_if_slides_and_master_match() {
     s.execute("design.background", &json!({"color": "#AA0000", "all": true})).unwrap();
     assert_eq!(undo_len(&s), n + 1, "now it is truly a no-op");
 }
+
+#[test]
+fn background_rejects_invalid_index() {
+    let mut s = Session::with_new();
+    s.execute("slide.new", &json!({})).unwrap();
+    s.execute("slide.go", &json!({"index": 0})).unwrap();
+    let before = s.doc().unwrap().doc.slides[0].background.clone();
+    for bad in [json!(-1), json!(1.5)] {
+        assert!(s.execute("design.background", &json!({"color": "#0000C8", "index": bad})).is_err());
+        assert!(s.execute("design.background", &json!({"transparency": 0.5, "index": bad})).is_err());
+        assert_eq!(s.doc().unwrap().doc.slides[0].background, before);
+    }
+}