diff --git a/crates/engine/src/cmd/prop.rs b/crates/engine/src/cmd/prop.rs
index b23e73575..761a295b2 100644
--- a/crates/engine/src/cmd/prop.rs
+++ b/crates/engine/src/cmd/prop.rs
@@ -1370,6 +1370,13 @@ fn convert_expr_to_keys(s: &mut Session, p: &Value) -> Result<Value> {
         if sel.is_empty() {
             return Err(bad(CMD, "no `path` given and no selected property has an expression"));
         }
+        // Every layer is checked before the first is converted: a refusal leaves them all as they were.
+        let step = comp.frame_duration().0;
+        for (lid, _) in &sel {
+            if let Some(l) = comp.layer(*lid) {
+                check_convertible(step, l)?;
+            }
+        }
         let mut n = 0;
         for (lid, uid) in sel {
             n += convert_one_expr(s, cid, lid, uid)?.as_u64().unwrap_or(0);
@@ -1380,14 +1387,36 @@ fn convert_expr_to_keys(s: &mut Session, p: &Value) -> Result<Value> {
     convert_one_expr(s, cid, lid, uid)
 }
 
+/// Most keyframes Convert Expression to Keyframes makes: 2^20, 4.8 hours at 60 fps (After Effects
+/// compositions run to 3 hours, 648,000 frames at 60 fps).
+const MAX_CONVERTED_KEYS: u64 = 1 << 20;
+const _: () = assert!(MAX_CONVERTED_KEYS >= 3 * 3600 * 60);
+
+/// One evaluation and one key per frame of the layer: its In and Out points and the composition's
+/// frame duration are saved data, so the step must advance and the count (a partial last frame
+/// rounded up, as the loop steps, in 128 bits) is checked before any key is made.
+fn check_convertible(step: i64, layer: &Layer) -> Result<()> {
+    const CMD: &str = "prop.convertExpressionToKeyframes";
+    if step <= 0 {
+        return Err(bad(CMD, "the composition's frame duration is not positive"));
+    }
+    let (step, span) = (i128::from(step), i128::from(layer.out_point.0) - i128::from(layer.in_point.0));
+    let frames = if span > 0 { (span + step - 1) / step } else { 0 };
+    if frames > i128::from(MAX_CONVERTED_KEYS) {
+        return Err(bad(CMD, format!("the layer spans {frames} frames; at most {MAX_CONVERTED_KEYS} are converted")));
+    }
+    Ok(())
+}
+
 /// Replaces the expression on one property with keyframes sampled from its evaluated values.
 /// Returns the number of properties converted.
 fn convert_one_expr(s: &mut Session, cid: ItemId, lid: LayerId, uid: Uid) -> Result<Value> {
     let comp = s.project.comp(cid).ok_or(EngineError::NoComp)?.clone();
     let layer = comp.layer(lid).ok_or(EngineError::NoComp)?.clone();
     let pr = layer.props.find(uid).ok_or_else(|| bad("prop.convertExpressionToKeyframes", "no property"))?.clone();
-    let mut keys = vec![];
     let fd = comp.frame_duration();
+    check_convertible(fd.0, &layer)?;
+    let mut keys = vec![];
     let mut t = layer.in_point;
     while t < layer.out_point {
         let ctx = effectcraft_render::EvalCtx {
diff --git a/crates/engine/src/tests/timeline.rs b/crates/engine/src/tests/timeline.rs
index b5a44239b..291ad884e 100644
--- a/crates/engine/src/tests/timeline.rs
+++ b/crates/engine/src/tests/timeline.rs
@@ -1581,3 +1581,28 @@ fn a_typed_component_moves_its_follow_components_by_each_layers_own_change() {
     let keys: Vec<Vec<f64>> = prop(&s, b, "audio/levels").keys.iter().map(|k| k.value.components()).collect();
     assert_eq!(keys, vec![vec![-20.0, -30.0], vec![-4.0, -34.0]], "B's 1 s key rises 6 dB on both channels");
 }
+
+#[test]
+fn convert_expression_to_keyframes_refuses_a_layer_of_millions_of_frames() {
+    let (mut s, l) = setup();
+    s.execute("prop.setExpression", json!({"layer": l, "path": "transform/rotation", "expression": "value"})).unwrap();
+    // A saved Out point a million seconds after the In point.
+    let cid = s.active_comp_id().unwrap();
+    std::sync::Arc::make_mut(&mut s.project).comp_mut(cid).unwrap().layer_mut(effectcraft_project::LayerId(l)).unwrap().out_point =
+        effectcraft_time::Tick::from_seconds_f64(1e6);
+    let e = s.execute("prop.convertExpressionToKeyframes", json!({"layer": l, "path": "transform/rotation"})).unwrap_err().to_string();
+    assert!(e.contains("at most"), "{e}");
+    assert!(prop(&s, l, "transform/rotation").keys.is_empty());
+}
+
+#[test]
+fn convert_expression_to_keyframes_refuses_a_zero_frame_duration() {
+    let (mut s, l) = setup();
+    s.execute("prop.setExpression", json!({"layer": l, "path": "transform/rotation", "expression": "value"})).unwrap();
+    // A saved frame rate of 0/1 s has a frame duration of 0: the sampling loop never advanced.
+    let cid = s.active_comp_id().unwrap();
+    std::sync::Arc::make_mut(&mut s.project).comp_mut(cid).unwrap().frame_rate.den = 0;
+    let e = s.execute("prop.convertExpressionToKeyframes", json!({"layer": l, "path": "transform/rotation"})).unwrap_err().to_string();
+    assert!(e.contains("frame duration"), "{e}");
+    assert!(prop(&s, l, "transform/rotation").keys.is_empty());
+}