diff --git a/src/geometry/arc.test.ts b/src/geometry/arc.test.ts
index d5adebe..260151e 100644
--- a/src/geometry/arc.test.ts
+++ b/src/geometry/arc.test.ts
@@ -32,4 +32,9 @@ describe('flattenArc', () => {
     const fine = flattenArc([10, 0], [0, 10], [0, 0], false, 0.01);
     expect(fine.length).toBeGreaterThan(coarse.length);
   });
+
+  it('returns a straight segment for a degenerate zero-radius arc', () => {
+    const pts = flattenArc([5, 5], [5, 5], [5, 5], false, 0.1);
+    expect(pts).toEqual([[5, 5], [5, 5]]);
+  });
 });
diff --git a/src/geometry/arc.ts b/src/geometry/arc.ts
index 326eb15..4b55238 100644
--- a/src/geometry/arc.ts
+++ b/src/geometry/arc.ts
@@ -16,6 +16,10 @@ export function flattenArc(
   clockwise: boolean,
   tolerance = 0.2,
 ): Vec2[] {
+  // Degenerate (zero-radius) arc: nothing to sweep, return a straight segment.
+  const r0 = Math.hypot(start[0] - center[0], start[1] - center[1]);
+  if (r0 < 1e-9) return [[start[0], start[1]], [end[0], end[1]]];
+
   const radius = Math.hypot(start[0] - center[0], start[1] - center[1]);
   const startAngle = Math.atan2(start[1] - center[1], start[0] - center[0]);
   const endAngle = Math.atan2(end[1] - center[1], end[0] - center[0]);
@@ -27,7 +31,7 @@ export function flattenArc(
     while (sweep <= 0) sweep += 2 * Math.PI; // sweep in (0, 2π]
   }
 
-  const maxStep = radius > tolerance ? 2 * Math.acos(1 - tolerance / radius) : Math.PI / 8;
+  const maxStep = radius > tolerance ? 2 * Math.acos(1 - tolerance / radius) : Math.PI / 8; // fallback step for tiny radii
   const steps = Math.max(1, Math.ceil(Math.abs(sweep) / maxStep));
 
   const pts: Vec2[] = [];