diff --git a/MultiTerm.Wpf.CustomControl/MultiFormatTextBox/MultiFormatTextBox.cs b/MultiTerm.Wpf.CustomControl/MultiFormatTextBox/MultiFormatTextBox.cs
index e44b8af..a799c9a 100644
--- a/MultiTerm.Wpf.CustomControl/MultiFormatTextBox/MultiFormatTextBox.cs
+++ b/MultiTerm.Wpf.CustomControl/MultiFormatTextBox/MultiFormatTextBox.cs
@@ -83,7 +83,7 @@ public class MultiFormatTextBox : Control
     ~MultiFormatTextBox()
     {
         // unregister events
-        if (this.CurrentMultiFormatString is INotifyCollectionChanged incc)
+        if (this.CurrentMultiFormatString != null && this.CurrentMultiFormatString is INotifyCollectionChanged incc)
         {
             incc.CollectionChanged -= this.MultiFormatString_CollectionChanged;
         }