diff --git a/crates/engine/src/cmd/annotate_tests.rs b/crates/engine/src/cmd/annotate_tests.rs
index 267510df7..b9985821d 100644
--- a/crates/engine/src/cmd/annotate_tests.rs
+++ b/crates/engine/src/cmd/annotate_tests.rs
@@ -341,3 +341,14 @@ fn hostile_annotation_params_never_panic() {
     let _ = cadcraft_render::build(s.doc().unwrap(), &cadcraft_doc::Space::Model, &cadcraft_render::Options::default());
     assert!(matches!(dim(&s, d).kind, DimKind::Linear { .. }));
 }
+
+#[test]
+fn mtext_rejects_nonpositive_height() {
+    let mut s = Session::new();
+    for height in [0.0, -2.0] {
+        assert!(s.execute("mtext", &json!({ "at": [0, 0], "text": "hi", "height": height })).is_err(), "height {height}");
+    }
+    assert!(s.doc().unwrap().model.is_empty());
+    s.execute("mtext", &json!({ "at": [0, 0], "text": "hi", "height": 2 })).unwrap();
+    assert_eq!(s.doc().unwrap().model.len(), 1);
+}
diff --git a/crates/engine/src/cmd/draw.rs b/crates/engine/src/cmd/draw.rs
index 9b90f8c35..188ea9edb 100644
--- a/crates/engine/src/cmd/draw.rs
+++ b/crates/engine/src/cmd/draw.rs
@@ -320,6 +320,9 @@ fn run_mtext(s: &mut Session, p: &Value) -> Result<Value> {
     let at = point_req("mtext", p, "at")?;
     let text = str_param(p, "text").ok_or_else(|| bad("mtext", "`text` is required"))?;
     let height = f64_or(p, "height", s.doc()?.header.f64("TEXTSIZE", 0.2));
+    if height <= 0.0 {
+        return Err(bad("mtext", "height must be positive"));
+    }
     let style = s.doc()?.header.str("TEXTSTYLE", "Standard");
     let k = EntityKind::MText(MText {
         insert: v3(at),