diff --git a/nonisotropic.cpp b/nonisotropic.cpp
index 73c0ab87..859ad836 100644
--- a/nonisotropic.cpp
+++ b/nonisotropic.cpp
@@ -1081,7 +1081,7 @@ EX namespace hybrid {
       cell *c1 = hybrid::get_where(c).first;
       wo = isize(cgi.shWall3D);
       int won = wo + c->type;
-      cgi.wallstart.pop_back();
+      if(!cgi.wallstart.empty()) cgi.wallstart.pop_back();
       cgi.reserve_wall3d(won);
       
       if(prod) for(int i=0; i<c1->type; i++) {