Windows.UI.Xaml.Markup.XamlParseException while trying to locate a string

I'm following this tutorial to create my first Windows app. I'm at step 3, and if I try to run the applications I get a Windows.UI.Xaml.Markup.XamlParseException here

<TextBlock x:Uid="messageLabel" x:Name="messageLabel" Text=""></TextBlock>

Since I'm not even changing the names from the guide, what is going on?