diff --git a/src/voronoi/_impl/_convex_intersection.scad b/src/voronoi/_impl/_convex_intersection.scad
index 9339b281..6ade2b6f 100644
--- a/src/voronoi/_impl/_convex_intersection.scad
+++ b/src/voronoi/_impl/_convex_intersection.scad
@@ -6,9 +6,10 @@ use <../../util/set/hashset_elems.scad>;
 include <../../__comm__/_pt2_hash.scad>;
 
 function _in_convex_r(s, convex_pts, pt, leng, i = 0) =
-    i + 1 == leng || 
-	s * cross(convex_pts[i] - pt, convex_pts[i + 1] - pt) > 0 && 
-    _in_convex_r(s, convex_pts, pt, leng, i + 1);
+    let(i1 = i + 1)
+    i1 == leng || 
+	s * cross(convex_pts[i] - pt, convex_pts[i1] - pt) > 0 && 
+    _in_convex_r(s, convex_pts, pt, leng, i1);
 
 function _in_convex(convex_pts, pt) = 
     let(leng = len(convex_pts))