diff --git a/main/Scene-load.cc b/main/Scene-load.cc
index 69b657e..a8d077a 100644
--- a/main/Scene-load.cc
+++ b/main/Scene-load.cc
@@ -569,6 +569,11 @@ refptr<Shape> Scene::processExtrude(refptr<Node> node)
         else if ( typeid(**it) == typeid(OffsetNode) )
         {
             double distance = (*it)->getNumber();
+            if (distance <= 0.0)
+            {
+                cerr << "Error: extrude distance must be positive" << endl;
+                exit(3);
+            }
             Vector scale(1, 1, 1);
             Vector position(0, 0, 0);
             for (Node_Iterator it2 = (*it)->getChildren().begin();
@@ -584,6 +589,11 @@ refptr<Shape> Scene::processExtrude(refptr<Node> node)
                     position = * (*it2)->getVector();
                 }
             }
+            if (scale[0] < 0.0 || scale[1] < 0.0)
+            {
+                cerr << "Error: extrude scale cannot be negative" << endl;
+                exit(3);
+            }
             extrude->addOffset(distance, scale, position);
         }
         else if ( (*it)->isMaterial() )
@@ -619,6 +629,11 @@ refptr<Polygon> Scene::processPolygon(refptr<Node> node)
             exit(3);
         }
     }
+    if (p->size() < 3)
+    {
+        cerr << "Error: Polygon with fewer than three points!" << endl;
+        exit(3);
+    }
     return p;
 }