diff --git a/bind/example.py b/bind/example.py
index 9ec268f..8a427b3 100644
--- a/bind/example.py
+++ b/bind/example.py
@@ -1,10 +1,24 @@
 
 from G4Calo import GeometryDescriptor, G4System
+import sys
 
 cw = GeometryDescriptor()
+
+
 for _ in range(25):
-    cw.addLayer(0.5, "G4Pb", False)
+    cw.addLayer(0.5, "G4_Pb", False)
     cw.addLayer(1.,"G4_POLYSTYRENE",True,1)
 
 #directly use G4System as a global singleton
 G4System.init(cw)
+
+
+df = G4System.run_batch(10, 'gamma', 1)
+cw = GeometryDescriptor()
+
+for _ in range(5):
+    cw.addLayer(0.5, "G4_Pb", False)
+    cw.addLayer(1.,"G4_POLYSTYRENE",True,1)
+
+G4System.init(cw)
+df = G4System.run_batch(10, 'gamma', 1)
\ No newline at end of file
diff --git a/include/GeometryDescriptor.hh b/include/GeometryDescriptor.hh
index 619ff42..54a4b99 100644
--- a/include/GeometryDescriptor.hh
+++ b/include/GeometryDescriptor.hh
@@ -134,6 +134,10 @@ public:
         }
     }
 
+    bool isEmpty()const{
+        return layers.size() == 0;
+    }
+
     void setG4System(G4System* g4system){
         this->g4system = g4system;
     }
diff --git a/src/G4System.cc b/src/G4System.cc
index 9bd61b6..9137f8f 100644
--- a/src/G4System.cc
+++ b/src/G4System.cc
@@ -14,6 +14,9 @@ void G4System::init(GeometryDescriptor &CW){
   if(CW.isAssigned()){
     throw std::runtime_error("GeometryDescriptor already assigned");
   }
+  if(CW.isEmpty()){
+    throw std::runtime_error("GeometryDescriptor is empty");
+  }
   if(assigned_cw != nullptr){
     //reassign
     assigned_cw->unAssign();