diff --git a/crates/engine/src/cmd/sketch_clipboard.rs b/crates/engine/src/cmd/sketch_clipboard.rs
index c9764455..364f1851 100644
--- a/crates/engine/src/cmd/sketch_clipboard.rs
+++ b/crates/engine/src/cmd/sketch_clipboard.rs
@@ -242,6 +242,10 @@ fn paste_step(extent: f64) -> f64 {
 
 fn paste(s: &mut Session, p: &Value) -> Result<Value> {
     let cmd = "sketch.paste";
+    // `p["sketch"] = …` below panics on anything but an object.
+    if !(p.is_object() || p.is_null()) {
+        return Err(bad(cmd, "params must be an object"));
+    }
     let Some(clip) = s.sketch_clipboard.clone() else { return Err(bad(cmd, "nothing copied (sketch.copy first)")) };
     let target = target_sketch(s, p, cmd)?;
     let positioned = p.get("at").is_some() || p.get("translate").is_some();
diff --git a/crates/engine/src/cmd/sketch_clipboard_tests.rs b/crates/engine/src/cmd/sketch_clipboard_tests.rs
index 86ec2e24..3399f442 100644
--- a/crates/engine/src/cmd/sketch_clipboard_tests.rs
+++ b/crates/engine/src/cmd/sketch_clipboard_tests.rs
@@ -231,3 +231,21 @@ fn cut_copies_and_deletes_in_one_step_and_pastes_back_in_place() {
     run(&mut s, "edit.undo", json!({}));
     assert_eq!(sketch(&s, sid), &before);
 }
+
+/// Parameters that are not a JSON object (a number, text, a list) are an error, not a panic:
+/// `sketch.cut` and `sketch.paste` add keys to a copy of the parameters, and serde_json panics
+/// when a key is set on anything but an object (#1536, found by `hostile_params_never_panic`).
+#[test]
+fn clipboard_commands_refuse_parameters_that_are_not_an_object() {
+    let (mut s, rect, _) = drawing();
+    run(&mut s, "sketch.copy", json!({"entities": [rect[0].clone()]}));
+    for id in ["sketch.cut", "sketch.copy", "sketch.paste"] {
+        for hostile in [json!(true), json!(-1), json!(1e308), json!("nope"), json!([]), json!([0, 0, 0]), json!([{"x": 1}])] {
+            let result = s.execute(id, &hostile);
+            assert!(!matches!(result, Err(crate::EngineError::Internal(..))), "{id} {hostile} panicked inside the command: {result:?}");
+            assert!(result.is_err(), "{id} {hostile} should be refused");
+        }
+    }
+    // Nothing was cut or pasted by the refused calls.
+    assert!(s.execute("sketch.paste", &json!({})).is_ok(), "the clipboard still holds the copy");
+}