Why am I getting a MissingMethodException when configuring nhibernate with fluent-nhibernate?

Following along with some tutorials, I have a .NET Core 2.2 project with nhibernate as the ORM. I have set up a SessionFactory class and upon running the app I get an exception:

FluentNHibernate.Cfg.FluentConfigurationException
  HResult=0x80131500
  Message=An invalid or incomplete configuration was used while creating a SessionFactory. Check PotentialReasons collection, and InnerException for more detail.


  Source=FluentNHibernate
  StackTrace:
   at FluentNHibernate.Cfg.FluentConfiguration.BuildSessionFactory()
   at Domain.SessionFactory.BuildSessionFactory(String connectionString) in D:\...\Domain\SessionFactory.cs:line 46
   at Domain.SessionFactory.Init(String connectionString) in D:\...\Domain\SessionFactory.cs:line 29
   at Application.Startup.Configure(IApplicationBuilder app, IHostingEnvironment env) in D:\...\.Application\Startup.cs:line 34

Inner Exception 1:
MissingMethodException: Method not found: 'System.Security.PermissionSet System.AppDomain.get_PermissionSet()'.

I have tried installing System.Security.Permissions, as well as System.AppDomain but no luck. I have also not found an answer in 3-ish hours searching the web. It seems to be when either of the lines AddFromAssembly or AddFromAssemblyOf<Entity>() are used (I don't yet understand what they're for, but when they are commented out I get no exception).

        private static ISessionFactory BuildSessionFactory(string connectionString)
        {
            var configuration = Fluently.Configure()
                .Database(MsSqlConfiguration.MsSql2012.ConnectionString(connectionString)
                .ShowSql())
                .Mappings(m => m.FluentMappings
                    .AddFromAssembly(Assembly.GetExecutingAssembly())
                    //.AddFromAssemblyOf<Entity>()
                    .Conventions.Add(
                        ForeignKey.EndsWith("Id"),
                        ConventionBuilder.Property
                            .When(criteria => criteria.Expect(x => x.Nullable, Is.Not.Set), x => x.Not.Nullable()))
                    .Conventions.Add<TableNameConvention>());

            //configuration.ExposeConfiguration(c =>
            //    new SchemaExport(c).Execute(true, true, false));

            return configuration.BuildSessionFactory();
        }

Turns out I needed to install nhibernate and fluentnhibernate packages in my application project as well for some reason.