diff --git a/geom/src/polygon.rs b/geom/src/polygon.rs
index 5c7ca2707b..682e7ecd67 100644
--- a/geom/src/polygon.rs
+++ b/geom/src/polygon.rs
@@ -146,7 +146,7 @@ impl Polygon {
 
     /// Top-left at the origin. Doesn't take Distance, because this is usually pixels, actually.
     pub fn maybe_rectangle(width: f64, height: f64) -> Result<Self> {
-        Ring::new(vec![
+        Ring::strict_new(vec![
             Pt2D::new(0.0, 0.0),
             Pt2D::new(width, 0.0),
             Pt2D::new(width, height),
diff --git a/geom/src/ring.rs b/geom/src/ring.rs
index e7abe61b98..5a39b09a92 100644
--- a/geom/src/ring.rs
+++ b/geom/src/ring.rs
@@ -43,6 +43,14 @@ impl Ring {
         Ok(result)
     }
 
+    pub fn strict_new(pts: Vec<Pt2D>) -> Result<Self> {
+        // Enforce no duplicate adjacent points
+        if let Some(pair) = pts.windows(2).find(|pair| pair[0] == pair[1]) {
+            bail!("Ring has duplicate adjacent points near {}", pair[0]);
+        }
+        Self::new(pts)
+    }
+
     pub fn must_new(pts: Vec<Pt2D>) -> Ring {
         Ring::new(pts).unwrap()
     }