diff --git a/grid.c b/grid.c
index 90a9278..04bb8a3 100644
--- a/grid.c
+++ b/grid.c
@@ -342,21 +342,23 @@ static int grid_edge_bydots_cmpfn(void *v1, void *v2)
     grid_edge *b = v2;
     grid_dot *da, *db;
 
-    /* Pointer subtraction is valid here, because all dots point into the
-     * same dot-list (g->dots).
-     * Edges are not "normalised" - the 2 dots could be stored in any order,
+    /* Edges are not "normalised" - the 2 dots could be stored in any order,
      * so we need to take this into account when comparing edges. */
 
     /* Compare first dots */
     da = (a->dot1 < a->dot2) ? a->dot1 : a->dot2;
     db = (b->dot1 < b->dot2) ? b->dot1 : b->dot2;
-    if (da != db)
-        return db - da;
+    if (da->index < db->index)
+        return -1;
+    if (da->index > db->index)
+        return +1;
     /* Compare last dots */
     da = (a->dot1 < a->dot2) ? a->dot2 : a->dot1;
     db = (b->dot1 < b->dot2) ? b->dot2 : b->dot1;
-    if (da != db)
-        return db - da;
+    if (da->index < db->index)
+        return -1;
+    if (da->index > db->index)
+        return +1;
 
     return 0;
 }