ASP.Net Identity how to set target DB?

ASP.Net Identity how to set target DB?

I am using the ASP.NET Identity Sample from the Asp-Team, and i am trying
to change the database for the IdentityDbContext...
I tried it with the constructor
public MyDbContext() : base("MyConnectionString") { } // base is
IdentityDbContext
like with a DbContext class.
That works well until i try to Register a User...
await IdentityStore.CreateLocalUser(user, model.Password)
returns false... no error, nothing.
Any ideas how to fix that?