diff --git a/src/Ironbug.HVAC/BaseClass/IB_ModelObject.cs b/src/Ironbug.HVAC/BaseClass/IB_ModelObject.cs
index 7072aa5d..7d3110da 100644
--- a/src/Ironbug.HVAC/BaseClass/IB_ModelObject.cs
+++ b/src/Ironbug.HVAC/BaseClass/IB_ModelObject.cs
@@ -508,7 +508,7 @@ protected T Duplicate<T>(Func<T> func) where T : IB_ModelObject
             newObj.Children.Clear();
             foreach (var child in this.Children)
             {
-                newObj.Children.Add(child.Duplicate());
+                newObj.Children.Add(child?.Duplicate());
             }
 
             newObj.IBProperties = this.IBProperties.Duplicate();