diff --git a/crates/engine/src/cmd/draw.rs b/crates/engine/src/cmd/draw.rs
index 538e53c5..430e47e3 100644
--- a/crates/engine/src/cmd/draw.rs
+++ b/crates/engine/src/cmd/draw.rs
@@ -780,6 +780,9 @@ impl Interactive for CircleM {
                 Ok(Step::Continue)
             }
             Input::Point(p) => {
+                if self.mode == 0 && self.pts.first().is_some_and(|c| c.near(p, 1e-12)) {
+                    return Err(crate::EngineError::Other("Value must be positive and nonzero.".into()));
+                }
                 self.pts.push(p);
                 let done = match self.mode {
                     0 if self.pts.len() == 2 => {
diff --git a/crates/engine/tests/draw_prompts.rs b/crates/engine/tests/draw_prompts.rs
index 9ddecdb4..4535b1be 100644
--- a/crates/engine/tests/draw_prompts.rs
+++ b/crates/engine/tests/draw_prompts.rs
@@ -44,6 +44,25 @@ fn spline_rejects_text_and_switches_method() {
     assert!(sp.fit.is_empty() && sp.control.len() == 4);
 }
 
+#[test]
+fn circle_radius_point_on_the_centre_is_refused() {
+    for input in ["circle 2,3 2,3", "circle 2,3 d 2,3"] {
+        let mut s = Session::new();
+        s.cmdline(input).unwrap();
+        assert_eq!(s.doc().unwrap().model.len(), 0, "{input}: no circle");
+        assert!(logged(&s, "Value must be positive and nonzero."), "{input}: {:?}", s.log);
+        assert!(s.running.is_some(), "{input}: still at the radius prompt");
+        s.cmdline("4").unwrap();
+        assert!(s.running.is_none(), "{input}: a positive value finishes");
+        assert_eq!(s.doc().unwrap().model.len(), 1, "{input}: one circle");
+        let want = if input.contains(" d ") { 2.0 } else { 4.0 };
+        assert!(
+            matches!(last(&s), EntityKind::Circle(c) if c.center.xy().near(Vec2::new(2.0, 3.0), 1e-9) && (c.radius - want).abs() < 1e-9),
+            "{input}: the centre is kept"
+        );
+    }
+}
+
 /// The two lowest vertices of a closed polyline share their y (the bottom edge is horizontal).
 fn bottom_y(k: &EntityKind) -> Option<f64> {
     let EntityKind::LwPolyline(p) = k else { return None };