Index: ext/rtree/rtree.c
==================================================================
--- ext/rtree/rtree.c
+++ ext/rtree/rtree.c
@@ -1953,16 +1953,16 @@
 /*
 ** Return the margin length of cell p. The margin length is the sum
 ** of the objects size in each dimension.
 */
 static RtreeDValue cellMargin(Rtree *pRtree, RtreeCell *p){
-  RtreeDValue margin;
-  int ii;
-  margin = DCOORD(p->aCoord[1]) - DCOORD(p->aCoord[0]);
-  for(ii=2; ii<pRtree->nDim2; ii+=2){
+  RtreeDValue margin = 0;
+  int ii = pRtree->nDim2 - 2;
+  do{
     margin += (DCOORD(p->aCoord[ii+1]) - DCOORD(p->aCoord[ii]));
-  }
+    ii -= 2;
+  }while( ii>=0 );
   return margin;
 }
 
 /*
 ** Store the union of cells p1 and p2 in p1.