You get this error, when you use neutral culture name to set culture of the application. 1 System.Globalization.CultureInfo ci = new System.Globalization.CultureInfo("tr"); 2 System.Threading.Thread.CurrentThread.CurrentCulture = ci; 3 System.Threading.Thread.CurrentThread.CurrentUICulture = ci; To prevent this error you can use explicit culture name like "tr-TR" or use the code below: 1 System.Globalization.CultureInfo ci = System.Globalization.CultureInfo.CreateSpecificCulture("tr"); Happy coding.