IdentityServer WithCustomUserService on External Login throws nullreference exception - identityserver3

Question

I Implementd a custom userservice to store user data in a database.
Since that I get a NullReferenceException when I try to authenticace with an external provider like facebook.
I can see this stack in the exception which indicates that a value is missing for the loginpage. As A result after clicking on the facebook button i'm standing again at the starting login page.
But I don't know why or which value exactly
I can see that at the end of AuthenticateExternal context.AuthenticateResult.User.Claims contains these claims
my user service looks simplified like this
public override async Task AuthenticateExternalAsync(ExternalAuthenticationContext context)
{
string id = context.ExternalIdentity.Claims.FirstOrDefault(i => i.Type == "id").Value;
var user = await gateway.ByExternalIds(context.ExternalIdentity.Provider, id);
if (user == null)
{
string displayName = context.ExternalIdentity.Claims.FirstOrDefault(i => i.Type.Equals("urn:facebook:name")).Value;
user = new User(context.ExternalIdentity);
await gateway.StoreAsync(user);
}
if (user != null)
{
await gateway.SetLastLogin(user.Subject, DateTimeOffset.Now);
context.AuthenticateResult = new AuthenticateResult(user.Subject, GetDisplayName(user), identityProvider: context.ExternalIdentity.Provider);
}
}
What am I missing?

Answer 1

Found it. This behavior occurs when the method for detemerning if the user ist active returns false.
Task IsActiveAsync(IsActiveContext context)
its little unexpected as I had expected that in this case i would see something like "unknown user" or "inactive user"