Can a C# method allow nullable lists as parameters?

I'm trying to write a method that allows for a list of Ids to search for, but I would like to allow the list to be optional. I've seen examples of List<string> but I'm having trouble with List<Guid>.

Trying this method in LinqPad, I get the message:

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.

Here's the method:

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();
}

UPDATE: Thanks to your comments, here's what I did:

public static ICollection<Project> GetProjectsAllowed
(
    this IMkpContext db,
    Guid profileId,
    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();
}