diff --git a/Polyhedron/demo/Polyhedron/Plugins/Isosurfacing_3/Isosurface_3_plugin.cpp b/Polyhedron/demo/Polyhedron/Plugins/Isosurfacing_3/Isosurface_3_plugin.cpp
index 975c8aac622..3e33a22f83d 100644
--- a/Polyhedron/demo/Polyhedron/Plugins/Isosurfacing_3/Isosurface_3_plugin.cpp
+++ b/Polyhedron/demo/Polyhedron/Plugins/Isosurfacing_3/Isosurface_3_plugin.cpp
@@ -270,7 +270,9 @@ private:
 
   Word_type image_data(int i, int j, int k)
   {
-    if ( i>=0 && i<image_->xdim() && j>=0 && j<image_->ydim() && k>=0 && k<image_->zdim() )
+    if ( i>=0 && static_cast<std::size_t>(i)<image_->xdim() &&
+         j>=0 && static_cast<std::size_t>(j)<image_->ydim() &&
+         k>=0 && static_cast<std::size_t>(k)<image_->zdim() )
       return image_->value(i, j, k);
     else
       return 0;
diff --git a/Polyhedron/demo/Polyhedron/Plugins/PCA/Scene_edit_box_item.cpp b/Polyhedron/demo/Polyhedron/Plugins/PCA/Scene_edit_box_item.cpp
index 78493723e31..0c4e1956c7d 100644
--- a/Polyhedron/demo/Polyhedron/Plugins/PCA/Scene_edit_box_item.cpp
+++ b/Polyhedron/demo/Polyhedron/Plugins/PCA/Scene_edit_box_item.cpp
@@ -973,7 +973,7 @@ void Scene_edit_box_item_priv::remodel_box(const QVector3D &dir)
   Q_FOREACH(Scene_edit_box_item::vertex*  selected_vertex, selected_vertices )
   {
     int id = selected_vertex->id;
-    CGAL_assume(id<8);
+    CGAL_assume(id<8 && id >=0);
     *selected_vertex->x = applyX(id, last_pool[id][0], dir.x());
     *selected_vertex->y = applyY(id, last_pool[id][1], dir.y());
     *selected_vertex->z = applyZ(id, last_pool[id][2], dir.z());