diff --git a/crates/cad/src/hpgl/write.rs b/crates/cad/src/hpgl/write.rs
index 22773ac68b..80ac40f866 100644
--- a/crates/cad/src/hpgl/write.rs
+++ b/crates/cad/src/hpgl/write.rs
@@ -142,9 +142,11 @@ impl Origin {
 #[derive(Clone, Debug)]
 pub struct HpglOptions {
     pub version: Version,
-    /// How far a flattened curve may lie from the curve, points ([`TOLERANCE_RANGE`]).
+    /// How far a flattened curve may lie from the curve, points ([`TOLERANCE_RANGE`]; a value
+    /// outside it is an error).
     pub tolerance: f64,
-    /// The pen table: pen 1's color first. Empty: a pen for each color the art draws in.
+    /// The pen table: pen 1's color first, up to [`MAX_PENS`] (more is an error). Empty: a pen for
+    /// each color the art draws in.
     pub pens: Vec<[u8; 3]>,
     pub origin: Origin,
     /// Draw each pen's paths nearest first, instead of in paint order.
@@ -184,8 +186,13 @@ pub fn export_regions(doc: &Document, opts: &HpglOptions, regions: &[Rect]) -> R
     if regions.iter().any(|r| ![r.x0, r.y0, r.x1, r.y1].iter().all(|v| v.is_finite()) || r.width() <= 0.0 || r.height() <= 0.0) {
         return Err("the export region is empty".into());
     }
-    let tolerance =
-        if opts.tolerance.is_finite() { opts.tolerance.clamp(*TOLERANCE_RANGE.start(), *TOLERANCE_RANGE.end()) } else { DEFAULT_TOLERANCE };
+    if !TOLERANCE_RANGE.contains(&opts.tolerance) {
+        return Err("the curve tolerance must be from 0.01 to 10 points".into());
+    }
+    if opts.pens.len() > MAX_PENS {
+        return Err("the pen table holds more than 255 pens".into());
+    }
+    let tolerance = opts.tolerance;
     // Layers the plot leaves out go first: baking can wrap one in art that would be drawn (its
     // crop marks).
     let shown = plotted(&doc.layers, false).map(|layers| {
diff --git a/crates/cad/src/hpgl/write/tests.rs b/crates/cad/src/hpgl/write/tests.rs
index 7ef5105340..0917cf02c9 100644
--- a/crates/cad/src/hpgl/write/tests.rs
+++ b/crates/cad/src/hpgl/write/tests.rs
@@ -441,6 +441,22 @@ fn an_empty_or_zero_length_plot_has_no_moves() {
     assert!(export(&d, &nan).is_err());
 }
 
+/// A tolerance outside its range and a pen table of more than 255 pens are errors, not changed.
+#[test]
+fn bad_options_are_errors() {
+    let d = three();
+    for tolerance in [0.0, 0.009, 10.01, f64::NAN, f64::INFINITY] {
+        let o = HpglOptions { tolerance, ..opts(&d) };
+        assert_eq!(export(&d, &o).err().as_deref(), Some("the curve tolerance must be from 0.01 to 10 points"), "{tolerance}");
+    }
+    for tolerance in [0.01, 10.0] {
+        assert!(export(&d, &HpglOptions { tolerance, ..opts(&d) }).is_ok(), "{tolerance}");
+    }
+    let o = HpglOptions { pens: vec![[0, 0, 0]; MAX_PENS + 1], ..opts(&d) };
+    assert_eq!(export(&d, &o).err().as_deref(), Some("the pen table holds more than 255 pens"));
+    assert!(export(&d, &HpglOptions { pens: vec![[0, 0, 0]; MAX_PENS], ..opts(&d) }).is_ok());
+}
+
 #[test]
 fn art_past_the_plotter_range_is_cut_off() {
     let d = doc_with(|d| vec![stroked(d, line((0.0, 50.0), (1e9, 50.0)), Color::BLACK)]);