Exception on casting to HandledMouseEventArgs

This should be simple but ... I have this event handler on a textbox to ignore right mouse clicks. When it executes I get InvalidCastException on the 3rd line.

        private void txtUserName_MouseDown(object sender, MouseEventArgs e) {
            if (e.Button == System.Windows.Forms.MouseButtons.Right) {
                HandledMouseEventArgs eh = (HandledMouseEventArgs)e;
                eh.Handled = true;
            }
        }

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Per http://stackoverflow.com/questions/4264579/how-to-prevent-mouse-scroll-in-toolstripcombobox and other examples, this ought to work. Instead, I get the exception. Ideas?