diff --git a/Source/ExcelDna.IntelliSense/UIMonitor/UIState.cs b/Source/ExcelDna.IntelliSense/UIMonitor/UIState.cs
index d24ed68..f34e752 100644
--- a/Source/ExcelDna.IntelliSense/UIMonitor/UIState.cs
+++ b/Source/ExcelDna.IntelliSense/UIMonitor/UIState.cs
@@ -186,7 +186,7 @@ internal FormulaEdit AsFormulaEdit()
         // Becomes a more general Window or Dialog to watch (for resize extension)
         public class SelectDataSource : UIState
         {
-            public IntPtr SelectDataSourceWindow;
+            public IntPtr SelectDataSourceWindow = IntPtr.Zero;
         }
 
         public override string ToString()