为什么WPF DataGrid在标题中显示类名？

I have created a custom style for my WPF datagrid by overriding its control template - nothing unusual, just copied the original template and modified it. Unfortunately, when the grid is drawn, the fully qualified class name of my ViewModel is showing up in the header (the ViewModel happens to be the DataContext of the UserControl that contains the DataGrid). Using Snoop, I have narrowed down which element in the template is showing this class name:I have created a custom style for my WPF datagr