C#方法可以允许可空列表作为参数吗？

在LinqPad中尝试这个方法,我收到消息：

Unable to create a null constant value of type ‘System.Collections.Generic.List`1[[System.Guid,mscorlib,Version=4.0.0.0,Culture=neutral,PublicKeyToken=b77a5c561934e089]]’. Only entity types,enumeration types or primitive types are supported in this context.

这是方法：

public static ICollection<Project> GetProjectsAllowed
(
    this IMkpContext db,Guid profileId,List<Guid> profOrgIds = null
)
{
    var projects = (from p in db.Project.Include(p => p.Proposals)
                    join po in db.ProfileOrganization on p.CreatedById equals po.ProfileId
                    where (profOrgIds == null || profOrgIds.Contains(po.OrganizationId))
                        && p.IsActive && po.IsActive
                    select p);

    return projects.ToList();
}

更新：感谢您的评论,这就是我的所作所为：

public static ICollection<Project> GetProjectsAllowed
(
    this IMkpContext db,List<Guid> profOrgIds = null,List<Guid> projectIds = null
)
{
    var projects = (from p in db.Project.Include(p => p.Proposals)
                    where p.IsActive
                    select p);

    if (profOrgIds != null && profOrgIds.Any())
    {
        var profileIds = db.ProfileOrganization
            .Where(po => po.IsActive && profOrgIds.Contains(po.OrganizationId))
            .Select(po => po.ProfileId);
        projects = projects.Where(p => profileIds.Contains(p.CreatedById));
    }

    if (projectIds != null && projectIds.Any())
        projects = projects.Where(proj => projectIds.Contains(proj.ProjectId));

    return projects.ToList();
}