IDE complaining that interface async methods are not implemented.

public interface INHibernateHelper
{
    Task SetUsername(User user, string? username, ISession? session = null);
    //...
}

public class NHibernateHelper : INHibernateHelper
{
    public async Task SetUsername(User user, string? username, ISession? session = null)
    {
        //...
    }
    //...
}