Can't add a claim with ASP.NET Core and identity

I try to add a custom claim to an existing Identity user but I get an exception at run-time:

Npgsql.PostgresException: 23502: null value in column "Id" violates not-null constraint

Help!

What I did. I've created a simple web app on windows using the following command line

dotnet new mvc --auth Individual --framework netcoreapp1.1

I made changes found here to make the app use PostgreSQL as the database back-end. The created default webapp works fine. I can register as a new user, login, log out, etc...

Then I modified the Test method of the Home controller (I know the exceptions are ugly):

    [Authorize]
    public async Task<IActionResult> Test()
    {
        var user = await GetCurrentUserAsync();
        if (user == null) {
            _logger.LogWarning("User is null.");
            throw new Exception("Not logged in");
        }
        _logger.LogWarning("User: {0}, {1}", user.Email, user);

        var claim = new Claim("TestClaimType", "TestClaimValue");

        IdentityResult idRes = IdentityResult.Failed();
        if (_userManager.SupportsUserClaim) {
            idRes = await _userManager.AddClaimAsync(user, claim); <------- Adding the claim
        }
        _logger.LogWarning("Return from adding claim");

        if (idRes != IdentityResult.Success) {
            throw new Exception("Failed to add claim.");
        }

        return View();
    }

After logging in, I trigger the Test method and get the following logging (the PostgresException is near the end):

info: Microsoft.AspNetCore.Hosting.Internal.WebHost[1]
      Request starting HTTP/1.1 GET http://localhost:5000/Home/Test
info: Microsoft.AspNetCore.Authentication.Cookies.CookieAuthenticationMiddleware[3]
      HttpContext.User merged via AutomaticAuthentication from authenticationScheme: Identity.Application.
info: Microsoft.AspNetCore.Authorization.DefaultAuthorizationService[1]
      Authorization was successful for user: [email protected].
info: Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker[1]
      Executing action method AlumniConnect.Controllers.HomeController.Test (AlumniConnect) with arguments ((null)) - ModelState is Valid
info: Microsoft.EntityFrameworkCore.Storage.IRelationalCommandBuilderFactory[1]
      Executed DbCommand (1ms) [Parameters=[@__get_Item_0='?'], CommandType='Text', CommandTimeout='30']
      SELECT "e"."Id", "e"."AccessFailedCount", "e"."ConcurrencyStamp", "e"."Email", "e"."EmailConfirmed", "e"."LockoutEnabled", "e"."LockoutEnd", "e"."NormalizedEmail", "e"."NormalizedUserName", "e"."PasswordHash", "e"."PhoneNumber", "e"."PhoneNumberConfirmed", "e"."SecurityStamp", "e"."TwoFactorEnabled", "e"."UserName"
      FROM "AspNetUsers" AS "e"
      WHERE "e"."Id" = @__get_Item_0
      LIMIT 1
warn: AlumniConnect.Controllers.HomeController[0]
      User: [email protected], [email protected]
warn: AlumniConnect.Controllers.HomeController[0]
      User: [email protected], [email protected]
info: Microsoft.EntityFrameworkCore.Storage.IRelationalCommandBuilderFactory[1]
      Executed DbCommand (4ms) [Parameters=[@__normalizedUserName_0='?'], CommandType='Text', CommandTimeout='30']
      SELECT "u"."Id", "u"."AccessFailedCount", "u"."ConcurrencyStamp", "u"."Email", "u"."EmailConfirmed", "u"."LockoutEnabled", "u"."LockoutEnd", "u"."NormalizedEmail", "u"."NormalizedUserName", "u"."PasswordHash", "u"."PhoneNumber", "u"."PhoneNumberConfirmed", "u"."SecurityStamp", "u"."TwoFactorEnabled", "u"."UserName"
      FROM "AspNetUsers" AS "u"
      WHERE "u"."NormalizedUserName" = @__normalizedUserName_0
      LIMIT 1
info: Microsoft.EntityFrameworkCore.Storage.IRelationalCommandBuilderFactory[1]
      Executed DbCommand (33ms) [Parameters=[@p0='?', @p1='?', @p2='?', @p17='?', @p3='?', @p4='?', @p18='?', @p5='?', @p6='?', @p7='?', @p8='?', @p9='?', @p10='?', @p11='?', @p12='?', @p13='?', @p14='?', @p15='?', @p16='?'], CommandType='Text', CommandTimeout='30']
      INSERT INTO "AspNetUserClaims" ("ClaimType", "ClaimValue", "UserId")
      VALUES (@p0, @p1, @p2)
      RETURNING "Id";
      UPDATE "AspNetUsers" SET "AccessFailedCount" = @p3, "ConcurrencyStamp" = @p4, "Email" = @p5, "EmailConfirmed" = @p6, "LockoutEnabled" = @p7, "LockoutEnd" = @p8, "NormalizedEmail" = @p9, "NormalizedUserName" = @p10, "PasswordHash" = @p11, "PhoneNumber" = @p12, "PhoneNumberConfirmed" = @p13, "SecurityStamp" = @p14, "TwoFactorEnabled" = @p15, "UserName" = @p16
      WHERE "Id" = @p17 AND "ConcurrencyStamp" = @p18;
fail: Microsoft.EntityFrameworkCore.DbContext[1]
      An exception occurred in the database while saving changes.
      Microsoft.EntityFrameworkCore.DbUpdateException: An error occurred while updating the entries. See the inner exception for details. ---> Npgsql.PostgresException: 23502: null value in column "Id" violates not-null constraint
         at Npgsql.NpgsqlConnector.<DoReadMessageAsync>d__6.MoveNext()
      --- End of stack trace from previous location where exception was thrown ---

There's lots of logging more but it doesn't seem to add new information. I see the same exception mentioned multiple times throughout the log.

What can I do? Is this a PostgreSQL specific issue? Am I trying to add a claim in the wrong way?

Thanks!