Why nullable objects must have a value?

I Write this code in ListHandlerfile.cs.

protected override void PrepareQuery(SqlQuery query)
{

    base.PrepareQuery(query);


    var PR = ProviderCredentialingFormRow.Fields;
    var connection = SqlConnections.NewByKey("Default");

    var user = connection.TryFirst<ProviderRegistrationRow>(ProviderRegistrationRow.Fields.UserId == User.GetIdentifier());
    var team = user.TeamId;

    query.Where(PR.TeamId == (Int32)team);
}

But I got this error Nullable object must have value. How I clear It?