diff --git a/src/main/java/de/neemann/digital/gui/draw/library/CustomElement.java b/src/main/java/de/neemann/digital/gui/draw/library/CustomElement.java
index 283da829f..a9701c8db 100644
--- a/src/main/java/de/neemann/digital/gui/draw/library/CustomElement.java
+++ b/src/main/java/de/neemann/digital/gui/draw/library/CustomElement.java
@@ -7,22 +7,25 @@ import de.neemann.digital.core.element.Element;
 import de.neemann.digital.gui.draw.elements.Circuit;
 import de.neemann.digital.gui.draw.elements.PinException;
 import de.neemann.digital.gui.draw.model.ModelDescription;
+import de.neemann.digital.gui.draw.model.NetList;
 
 /**
  * @author hneemann
  */
 public class CustomElement implements Element {
 
-    private Circuit circuit;
-    private ElementLibrary library;
+    private final NetList netList;
+    private final Circuit circuit;
+    private final ElementLibrary library;
 
     public CustomElement(Circuit circuit, ElementLibrary library) {
         this.circuit = circuit;
         this.library = library;
+        netList = new NetList(circuit.getWires());
     }
 
     public ModelDescription getModelDescription() throws PinException, NodeException {
-        return new ModelDescription(circuit, library, true);
+        return new ModelDescription(circuit, library, true, new NetList(netList));
     }
 
     @Override
diff --git a/src/main/java/de/neemann/digital/gui/draw/model/ModelDescription.java b/src/main/java/de/neemann/digital/gui/draw/model/ModelDescription.java
index 8476bf448..fc1ccb5d2 100644
--- a/src/main/java/de/neemann/digital/gui/draw/model/ModelDescription.java
+++ b/src/main/java/de/neemann/digital/gui/draw/model/ModelDescription.java
@@ -37,10 +37,13 @@ public class ModelDescription implements Iterable<ModelEntry> {
     public ModelDescription(Circuit circuit, ElementLibrary library) throws PinException, NodeException {
         this(circuit, library, false);
     }
-
     public ModelDescription(Circuit circuit, ElementLibrary library, boolean readAsCustom) throws PinException, NodeException {
+        this(circuit, library, readAsCustom, new NetList(circuit.getWires()));
+    }
+
+    public ModelDescription(Circuit circuit, ElementLibrary library, boolean readAsCustom, NetList netList) throws PinException, NodeException {
+        this.netList = netList;
         entries = new ArrayList<>();
-        netList = new NetList(circuit.getWires());
         if (readAsCustom)
             ioMap = new HashMap<>();
         else
@@ -73,7 +76,6 @@ public class ModelDescription implements Iterable<ModelEntry> {
 
             for (Pin p : pins)
                 netList.add(p);
-
         }
 
 
diff --git a/src/main/java/de/neemann/digital/gui/draw/model/Net.java b/src/main/java/de/neemann/digital/gui/draw/model/Net.java
index 4d2b2530f..248c1469c 100644
--- a/src/main/java/de/neemann/digital/gui/draw/model/Net.java
+++ b/src/main/java/de/neemann/digital/gui/draw/model/Net.java
@@ -24,6 +24,12 @@ public class Net {
     private final ArrayList<Pin> pins;
     private final ArrayList<Wire> wires;
 
+    public Net(Net toCopy) {
+        points = toCopy.points;  // no deep copy of points necessary
+        wires = null;            // wires not needed
+        pins = new ArrayList<>(toCopy.pins); // Pins are changed so create a deep copy
+    }
+
     public Net(Wire w) {
         points = new HashSet<>();
         points.add(w.p1);
@@ -87,14 +93,15 @@ public class Net {
         for (Pin i : inputs)
             i.setValue(value);
 
-        if (bindWiresToValues)
+        if (bindWiresToValues && wires != null)
             for (Wire w : wires)
                 w.setValue(value);
     }
 
     public void setHighLight(boolean highLight) {
-        for (Wire w : wires)
-            w.setHighLight(highLight);
+        if (wires != null)
+            for (Wire w : wires)
+                w.setHighLight(highLight);
     }
 
     public boolean containsValue(ObservableValue v) {
diff --git a/src/main/java/de/neemann/digital/gui/draw/model/NetList.java b/src/main/java/de/neemann/digital/gui/draw/model/NetList.java
index 8d711df4f..74957bc25 100644
--- a/src/main/java/de/neemann/digital/gui/draw/model/NetList.java
+++ b/src/main/java/de/neemann/digital/gui/draw/model/NetList.java
@@ -21,6 +21,13 @@ public class NetList implements Iterable<Net> {
             add(w);
     }
 
+    public NetList(NetList toCopy) {
+        netList = new ArrayList<>();
+        for (Net net : toCopy)
+            netList.add(new Net(net));
+    }
+
+
     public void add(NetList netList) {
         this.netList.addAll(netList.netList);
     }