diff --git a/crates/engine/src/cmd/edit.rs b/crates/engine/src/cmd/edit.rs
index 2c767b93..35930cb7 100644
--- a/crates/engine/src/cmd/edit.rs
+++ b/crates/engine/src/cmd/edit.rs
@@ -1177,7 +1177,7 @@ fn paint_list(s: &mut Session, p: &Value, fills: bool, add: bool) -> Result<Valu
 fn paint_set(s: &mut Session, p: &Value, fills: bool) -> Result<Value> {
     let cmd = if fills { "fill.set" } else { "stroke.set" };
     let ids = req_targets(cmd, s, p)?;
-    let idx = num(p, "index").map(|x| x.max(0.0) as usize);
+    let idx = if get(p, "index").is_none() { None } else { Some(req_index(cmd, p, "index")?) };
     let new_paint = match get(p, "paint") {
         Some(v) => Some(paint_of(v).ok_or_else(|| bad(cmd, "invalid paint"))?),
         None => None,
diff --git a/crates/engine/src/lib.rs b/crates/engine/src/lib.rs
index 34653bd5..a84ca21b 100644
--- a/crates/engine/src/lib.rs
+++ b/crates/engine/src/lib.rs
@@ -1594,6 +1594,8 @@ mod page_copy_navigation_tests;
 #[cfg(test)]
 mod page_target_tests;
 #[cfg(test)]
+mod paint_index_tests;
+#[cfg(test)]
 mod paint_reorder_tests;
 #[cfg(test)]
 mod paint_tests;
diff --git a/crates/engine/src/paint_index_tests.rs b/crates/engine/src/paint_index_tests.rs
new file mode 100644
index 00000000..eddea306
--- /dev/null
+++ b/crates/engine/src/paint_index_tests.rs
@@ -0,0 +1,39 @@
+//! `fill.set` / `stroke.set` reject a negative, fractional or non-finite `index` instead of
+//! clamping or truncating it onto another paint (like `fill.remove` / `stroke.remove`).
+
+use figcraft_doc::NodeId;
+use serde_json::{Value, json};
+
+use crate::Session;
+
+fn ex(s: &mut Session, cmd: &str, p: Value) -> Value {
+    s.execute(cmd, &p).unwrap_or_else(|e| panic!("{cmd} {p}: {e}"))
+}
+
+fn paints(s: &Session, id: NodeId) -> String {
+    let n = s.doc().unwrap().node(id).unwrap();
+    format!("{:?} {:?}", n.fills, n.strokes)
+}
+
+#[test]
+fn paint_set_rejects_bad_index_and_leaves_the_paints_alone() {
+    let mut s = Session::new();
+    let r = NodeId(ex(&mut s, "shape.rectangle", json!({"x": 0, "y": 0, "width": 10, "height": 10}))["id"].as_u64().unwrap());
+    for add in ["fill.add", "stroke.add"] {
+        for c in ["#0000FF", "#FFFF00"] {
+            ex(&mut s, add, json!({"ids": [r], "color": c}));
+        }
+    }
+    let before = paints(&s, r);
+    for cmd in ["fill.set", "stroke.set"] {
+        for bad in [json!(-1), json!(0.5), json!(1.9), json!("-1"), json!("1.5"), json!("NaN"), json!("inf"), json!("abc")] {
+            let res = s.execute(cmd, &json!({"ids": [r], "index": bad, "color": "#FF0000"}));
+            assert!(res.is_err(), "{cmd} index {bad} was accepted: {res:?}");
+            assert_eq!(paints(&s, r), before, "{cmd} index {bad} changed the document");
+        }
+        ex(&mut s, cmd, json!({"ids": [r], "index": 1, "color": "#FF0000"}));
+        assert_ne!(paints(&s, r), before, "{cmd} with a valid index still edits");
+        ex(&mut s, "edit.undo", json!({}));
+        assert_eq!(paints(&s, r), before);
+    }
+}