Found markup element with unexpected name 'Cascading AuthenticationState'

I am getting this error on my App.razor:

Found markup element with unexpected name'CascadingAuthenticationState'. If this is intended to be a component,add a @using directive for it namespace

This is the code I am using

<CascadingAuthenticationState><Router AppAssembly="@typeof(Program).Assembly"><Found Context="routeData"><RouteView RouteData="@routeData" DefaultLayout="@typeof(MainLayout)" /></Found><NotFound><LayoutView Layout="@typeof(MainLayout)"><p>Sorry, there's nothing at this address.</p></LayoutView></NotFound></Router></CascadingAuthenticationState>

I am using Visual Studio 2019 preview and I can run the application but, why do I have the red line in the Cascading....?