diff --git a/xs/src/admesh/normals.c b/xs/src/admesh/normals.c
index 843a25d16a..b4a0166d0b 100644
--- a/xs/src/admesh/normals.c
+++ b/xs/src/admesh/normals.c
@@ -142,7 +142,8 @@ stl_fix_normal_directions(stl_file *stl) {
         }
       }
       /* If this edge of the facet is connected: */
-      if(stl->neighbors_start[facet_num].neighbor[j] != -1) {
+      if(stl->neighbors_start[facet_num].neighbor[j] != -1 &&
+         stl->neighbors_start[facet_num].neighbor[j] < stl->stats.number_of_facets*sizeof(char)) {
         /* If we haven't fixed this facet yet, add it to the list: */
         if(norm_sw[stl->neighbors_start[facet_num].neighbor[j]] != 1) {
           /* Add node to beginning of list. */