diff --git a/crates/engine/src/cmd/freeform.rs b/crates/engine/src/cmd/freeform.rs
index 21fd5c92..7d984a0f 100644
--- a/crates/engine/src/cmd/freeform.rs
+++ b/crates/engine/src/cmd/freeform.rs
@@ -148,9 +148,19 @@ pub(crate) fn parse_freeform(v: &Value) -> std::result::Result<Freeform, String>
         point_fields(pv, &mut pt, Affine::IDENTITY).map_err(|e| format!("freeform point {i}: {e}"))?;
         f.points.push(pt);
     }
-    for l in v.get("lines").and_then(Value::as_array).into_iter().flatten() {
-        let ix = l.as_array().map(|a| a.iter().filter_map(Value::as_u64).map(|i| i as usize).collect()).unwrap_or_default();
-        f.add_line(ix)?;
+    if let Some(lines) = v.get("lines") {
+        let lines = lines.as_array().ok_or("`freeform.lines` must be an array")?;
+        for (line, values) in lines.iter().enumerate() {
+            let values = values.as_array().ok_or_else(|| format!("freeform line {line} must be an array of point indices"))?;
+            let indices: Vec<usize> = values
+                .iter()
+                .map(|v| {
+                    let id = v.as_u64().ok_or_else(|| format!("freeform line {line} has a non-integer point index: {v}"))?;
+                    usize::try_from(id).map_err(|_| format!("freeform line {line} has an out-of-range point index: {id}"))
+                })
+                .collect::<std::result::Result<_, _>>()?;
+            f.add_line(indices).map_err(|e| format!("freeform line {line}: {e}"))?;
+        }
     }
     if let Some(m) = str_param(v, "mode") {
         f.mode = parse_mode(m)?;
@@ -323,3 +333,30 @@ fn get(s: &mut Session, p: &Value) -> Result<Value> {
     v["selected"] = json!(selected);
     Ok(v)
 }
+
+
+#[cfg(test)]
+mod parse_tests {
+    use super::*;
+
+    #[test]
+    fn malformed_freeform_lines_are_not_silently_shortened() {
+        let points = json!([
+            {"at": [0, 0], "color": "#ff0000"},
+            {"at": [50, 0], "color": "#00ff00"},
+            {"at": [100, 0], "color": "#0000ff"}
+        ]);
+        for lines in [
+            json!([[0, "broken", 2]]),
+            json!([[0, -1, 2]]),
+            json!([[0, 1.5, 2]]),
+            json!([{"points": [0, 1]}]),
+            json!("not an array"),
+        ] {
+            let value = json!({"points": points, "lines": lines, "mode": "lines"});
+            assert!(parse_freeform(&value).is_err(), "invalid lines accepted: {value}");
+        }
+        let good = parse_freeform(&json!({"points": points, "lines": [[0, 1, 2]], "mode": "lines"})).unwrap();
+        assert_eq!(good.lines, vec![vec![0, 1, 2]]);
+    }
+}