Visual Studio 2013 is rewriting my FontFamily binding to "Global User Interface"

Whenever I save my XAML page in a C # Windows Store App project, the value for my property FontFamily

for is TextBlock

overwritten from:

FontFamily="{Binding SettingsViewModel.Font}"

      

        
        
        
      

    

to

FontFamily="Global User Interface"

      

        
        
        
      

    

What can I do for this?