diff --git a/crates/cad/src/tests.rs b/crates/cad/src/tests.rs
index 996f6fba..37bebbd0 100644
--- a/crates/cad/src/tests.rs
+++ b/crates/cad/src/tests.rs
@@ -125,6 +125,18 @@ fn every_version_declares_its_acadver_and_is_well_formed() {
     assert_eq!(DxfVersion::from_id("2019"), None);
 }
 
+#[test]
+fn r12_viewport_has_no_duplicate_group_codes() {
+    let d = shapes_doc();
+    let out = export(&d, &DxfOptions { version: DxfVersion::R12, ..opts(&d) }).unwrap();
+    let vport = table(&out, "VPORT").into_iter().find(|o| kind(o) == "VPORT").expect("VPORT table record");
+    let codes: Vec<i32> = vport.iter().map(|pair| pair.0).collect();
+    let unique: std::collections::HashSet<_> = codes.iter().collect();
+    assert_eq!(unique.len(), codes.len(), "R12 VPORT record has duplicate group codes: {vport:?}");
+    let z = vport.iter().find(|pair| pair.0 == 36).map(|pair| pair.1.trim().parse::<f64>().unwrap());
+    assert_eq!(z, Some(1.0), "the view direction looks down the z axis");
+}
+
 #[test]
 fn lines_are_lwpolylines_curves_splines_and_fills_hatches() {
     let d = shapes_doc();
diff --git a/crates/cad/src/writer.rs b/crates/cad/src/writer.rs
index 2fe4ec39..665f8576 100644
--- a/crates/cad/src/writer.rs
+++ b/crates/cad/src/writer.rs
@@ -441,7 +441,8 @@ impl Drawing {
         c.xy(13, 0.0, 0.0);
         c.xy(14, 1.0, 1.0);
         c.xy(15, 10.0, 10.0);
-        c.xyz(16, 0.0, 0.0);
+        // The view direction looks down the z axis: (0, 0, 1), each code once.
+        c.xy(16, 0.0, 0.0);
         c.num(36, 1.0);
         c.xyz(17, 0.0, 0.0);
         let (w, h) = ((x1 - x0).abs(), (y1 - y0).abs());