Index: ext/rtree/geopoly.c
==================================================================
--- ext/rtree/geopoly.c
+++ ext/rtree/geopoly.c
@@ -525,11 +525,11 @@
 ){
   GeoPoly *p = geopolyFuncParam(context, argv[0], 0);
   if( p ){
     if( geopolyArea(p)<0.0 ){
       int ii, jj;
-      for(ii=2, jj=p->nVertex*2 - 4; ii<jj; ii+=2, jj-=2){
+      for(ii=2, jj=p->nVertex*2 - 2; ii<jj; ii+=2, jj-=2){
         GeoCoord t = p->a[ii];
         p->a[ii] = p->a[jj];
         p->a[jj] = t;
         t = p->a[ii+1];
         p->a[ii+1] = p->a[jj+1];