ASP.Net Identity not creating user

I've used ASP.Net Identity a couple of times now. On a new project I seem to be having an issue creating a user.

When calling _userManager.Create() I get the following error.

  The string '{ Name: IX_UserId, Order: 0 }' was not 
  in the expected format to be deserialized by the 
  IndexAnnotationSerializer. Serialized values are expected to have 
  the format '{ Name: 'MyIndex', Order: 7, IsClustered: True, 
               sUnique: False } { } { Name: 'MyOtherIndex' }'.

I've tried using the following DbContext, which - apart from the class name - is identical to the DbContext i have in another project, that works

public partial class ISIdentityDbContext : IdentityDbContext<IdentityUser>
    {

        public ISIdentityDbContext()
            : base("ISIdentityDbContext")
        { }

        public DbSet<ApplicationUserUserInfoMap> ApplicationUserUserInfoMap { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            base.OnModelCreating(modelBuilder);

            // asp.net identity - call the tables something else..
            modelBuilder.Entity<IdentityRole>().ToTable("ApplicationRoles");
            modelBuilder.Entity<IdentityUserClaim>().ToTable("ApplicationUserClaims");
            modelBuilder.Entity<IdentityUserLogin>().ToTable("ApplicationUserLogins");
            modelBuilder.Entity<IdentityUserRole>().ToTable("ApplicationUserRoles");
            modelBuilder.Entity<IdentityUser>().ToTable("ApplicationUser");  
        }              

    }

I have tried the following:

 using (ISIdentityDbContext context = new ISIdentityDbContext())
            {                
                _userManager = new UserManager<IdentityUser>(new UserStore<IdentityUser>(context));
                IdentityUser user = new IdentityUser();
                user.UserName = "darren";
                _userManager.Create(user, "password");
            }

And also, the one I really need to get working as it's extending the ApplicationUser (IdentityUser)

using (ISIdentityDbContext context = new ISIdentityDbContext())
            {
                _userManager = new UserManager<LegacyApplicationUser>(new UserStore<LegacyApplicationUser>(context));
                ApplicationUserUserInfoMap map = new ApplicationUserUserInfoMap();
                map.UserGUID = "anIdFromAnotherTable";

                LegacyApplicationUser user = new LegacyApplicationUser();
                user.UserInfoMap = map;
                user.UserName = "darren";
                _userManager.Create(user, "password");
            }

Where my LegacyApplicationUser is:

   public class LegacyApplicationUser : IdentityUser
    {
        public virtual ApplicationUserUserInfoMap UserInfoMap { get; set; }
    }

    public class ApplicationUserUserInfoMap
    {
        public int ID { get; set; }
        public string UserGUID { get; set; }
    }

I'm totally stumped...no matter whether i rebuild my database to match the standard Identity users or use my extended version i keep getting the same exception shown at the top.

I've likely missed something, though can't figure what....

Any ideas?