Asp.Net Core 1.0.0: Npgsql.EntityFrameworkCore.PostgreSQL Migration error

My Startup.cs:

    services.AddEntityFrameworkNpgsql()
    .AddDbContext<ApplicationDbContext>(o =>
        o.UseNpgsql(Configuration["Data:DefaultConnection:ConnectionString"]
    ));

My project.json:

"Npgsql.EntityFrameworkCore.PostgreSQL": "1.0.0",
"Microsoft.EntityFrameworkCore": "1.0.0",

My project's dotnet --version:

1.0.0-preview2-003121

Then dotnet ef migrations add Firstsays

The current CSharpMigrationOperationGenerator cannot scaffold operations of type 'Microsoft.EntityFrameworkCore.Migrations.Operations.NpgsqlCreatePostgresExtensionOperation'. Configure your services to use one that can.

How can I specify an appropriate generator? Or maybe I'm doing wrong somewhere? Thank you in advance