I was considering writing up a full list of steps to reproduce the issue, however I found this webpage which provides a perfect example and is well documented:
http://www.c-sharpcorner.com/UploadFile/pchandraker/1066/
An exception of type System.Windows.Markup.XamlParseException is thrown with the message AG_E_PARSER_UNKNOWN_TYPE.
An exception should not be thrown, the strings should be visible during design-time with Visual Studio, and available at runtime.