QueryException: could not resolve property

Okay, so I am pretty green to Fluent NHibernate, but it's been great getting to know it. However, I am hitting a stumbling point and nee help. I am attempting a Linq query trying to get Cases tied to a logged-in User, and it involves going through an obvious CaseUser table. What am I missing? (BTW, any other critique is welcome.) I am using FluentNHibernate dll v1.1.0.685, NHibernate v2.1.2.4000, NHibernate.Linq.dll v1.1.0.1001 and VS 2008.

The first three lines of the error are:

[QueryException: could not resolve property: User of: Project.Domain.Case]
NHibernate.Persister.Entity.AbstractPropertyMapping.ToType(String propertyName) +115
NHibernate.Persister.Entity.AbstractEntityPersister.ToType(String propertyName) +44
NHibernate.Loader.Criteria.EntityCriteriaInfoProvider.GetType(String relativePath) +40

The exact statement that is failing is:

public override Expression<Func<Case, bool>> MatchingCriteria
{
    get { return u => u.CaseUsers.Where(x => (x.User.UserName == this.userName)).Count() > 0; }
}

Here's the Case class. (Statement in comments is work-in-progress):

public class Case : IntegerKeyEntity
{
    private ICollection<Activity> activities = new HashSet<Activity>();
    private ICollection<CaseUser> caseUsers = new HashSet<CaseUser>();
    //private ICollection<Feature> caseFeatures = new HashSet<CaseFeature>();

    public virtual Patient Patient { get; set; }
    public virtual string CaseNb { get; set; }
    public virtual bool IsDeleted { get; set; }
    public virtual DateTime CreateDt { get; set; }
    public virtual DateTime? UpdateDt { get; set; }

    public virtual IEnumerable<Activity> Activities { get { return activities; } }
    public virtual IEnumerable<CaseUser> CaseUsers { get { return caseUsers; } }
    //public virtual IEnumerable<Feature> CaseFeatures { get { return caseFeatures; } }
}

Here's the CaseUser:

public class CaseUser : IntegerKeyEntity
{
    [EntityKeyAttribute]
    public virtual Case Case { get; set; }
    [EntityKeyAttribute]
    public virtual User User { get; set; }
    [EntityKeyAttribute]
    public virtual CaseUserType CaseUserType { get; set; }
    public virtual DateTime? FromDt { get; set; }
    public virtual DateTime? ThruDt { get; set; }
    public virtual DateTime CreateDt { get; set; }
    public virtual DateTime? UpdateDt { get; set; }
    public virtual bool? IsDeleted { get; set; }
}

And, User:

public class User : IntegerKeyEntity

{
    //private ICollection<Audit> audits = new HashSet<Audit>();
    private ICollection<CaseUser> caseUsers = new HashSet<CaseUser>();
    //private ICollection<UserContact> userContacts = new HashSet<UserContact>();
    //private ICollection<UserFeature> userFeatures = new HashSet<UserFeature>();
    private ICollection<Group> groups = new HashSet<Group>(); 

    public virtual Account Account { get; set; }
    public virtual string UserName { get; set; }
    public virtual UserType UserType { get; set; }
    public virtual string LastName { get; set; }
    public virtual string MiddleName { get; set; }
    public virtual string FirstName { get; set; }
    public virtual string Suffix { get; set; }
    public virtual byte[] Password { get; set; }
    public virtual byte[] PwdSalt { get; set; }
    public virtual byte[] Signword { get; set; }
    public virtual byte[] SignSalt { get; set; }
    public virtual DateTime CreateDt { get; set; }
    public virtual DateTime? UpdateDt { get; set; }
    public virtual DateTime FromDt { get; set; }
    public virtual DateTime? ThruDt { get; set; }
    public virtual DateTime? LastLoginDt { get; set; }
    public virtual int? FailedLoginCount { get; set; }
    public virtual DateTime? LastPwdChangeDt { get; set; }
    public virtual DateTime? LastSwdChangeDt { get; set; }
    public virtual string Comment { get; set; }
    public virtual bool IsLockedOut { get; set; }
    public virtual bool IsDeleted { get; set; }
    public virtual string PasswordQuestion { get; set; }
    public virtual string PasswordAnswer { get; set; }
    public virtual byte[] SiteKeyImage { get; set; }
    public virtual string SiteKeyCaption { get; set; }
    public virtual bool? UserMustUpdatePwd { get; set; }

    //public virtual IEnumerable<Audit> Audits { get { return _caseUserList; } }
    public virtual IEnumerable<CaseUser> CaseUsers { get { return caseUsers; } }
    //public virtual IEnumerable<UserContact> UserContacts { get { return userContacts; } }
    //public virtual IEnumerable<UserFeature> UserFeatures { get { return userFeatures; } }
    public virtual IEnumerable<Group> Groups { get { return groups; } }

    public virtual bool IsUserInactive()
    {
        DateTime today = System.DateTime.Now;
        return (IsLockedOut || IsDeleted || (today < FromDt) || (today > (ThruDt.HasValue ? ThruDt.Value : System.DateTime.MaxValue)));
    }
}

Mappings:

public class CaseMap : ClassMap<Case>
{
    public CaseMap()
    {
        Id(x => x.Id, "CaseId");

        Map(x => x.CaseNb)
            .Length(32)
            .Nullable();
        Map(x => x.IsDeleted)
            .Not.Nullable();
        Map(x => x.CreateDt)
            .Not.Nullable();
        Map(x => x.UpdateDt)
            .Nullable();

        References(x => x.Patient)
            .Class(typeof(Patient))
            .Not.Nullable()
            .Column("PatientId");

        HasMany(x => x.CaseUsers)
            .KeyColumn("CaseId")
            .Access.CamelCaseField(Prefix.None)
            .Cascade.All()
            .AsSet();
        HasMany(x => x.Activities)
            .KeyColumn("CaseId")
            .Access.CamelCaseField(Prefix.None)
            .Cascade.All()
            .AsSet();

    }
}

public class CaseUserMap : ClassMap<CaseUser>
{
    public CaseUserMap()
    {
        Id(x => x.Id, "CaseUserId");

        Map(x => x.FromDt)
            .Nullable();
        Map(x => x.ThruDt)
            .Nullable();
        Map(x => x.CreateDt)
            .Not.Nullable();
        Map(x => x.UpdateDt)
            .Nullable();
        Map(x => x.IsDeleted)
            .Nullable();

        References(x => x.Case)
            .Class(typeof(Case))
            .Not.Nullable()
            .Column("CaseId"); 
        References(x => x.User)
            .Class(typeof(User))
            .Not.Nullable()
            .Column("UserId");
        References(x => x.CaseUserType)
            .Class(typeof(CaseUserType))
            .Not.Nullable()
            .Column("CaseUserTypeId");

    }
}

public class UserMap : ClassMap<User>
{
    public UserMap()
    {
        Id(x => x.Id, "UserId");

        Map(x => x.UserName)
            .Length(32)
            .Not.Nullable();
        Map(x => x.LastName)
            .Length(128)
            .Not.Nullable();
        Map(x => x.MiddleName)
            .Length(128)
            .Nullable();
        Map(x => x.FirstName)
            .Length(128)
            .Nullable();
        Map(x => x.Suffix)
            .Length(12)
            .Nullable();
        Map(x => x.Password)
            .Not.Nullable();
        Map(x => x.PwdSalt)
            .Not.Nullable();
        Map(x => x.Signword)
            .Nullable();
        Map(x => x.SignSalt)
            .Nullable();
        Map(x => x.CreateDt)
            .Not.Nullable();
        Map(x => x.UpdateDt)
            .Nullable();
        Map(x => x.FromDt)
            .Nullable();
        Map(x => x.ThruDt)
            .Nullable();
        Map(x => x.LastLoginDt)
            .Nullable();
        Map(x => x.FailedLoginCount)
            .Nullable();
        Map(x => x.LastPwdChangeDt)
            .Nullable();
        Map(x => x.LastSwdChangeDt)
            .Nullable();
        Map(x => x.Comment)
            .Length(512)
            .Nullable();
        Map(x => x.IsLockedOut)
            .Not.Nullable();
        Map(x => x.IsDeleted)
            .Not.Nullable();
        Map(x => x.PasswordQuestion)
            .Length(256)
            .Not.Nullable();
        Map(x => x.PasswordAnswer)
            .Length(128)
            .Not.Nullable();
        Map(x => x.SiteKeyImage)
            .CustomSqlType("VARBINARY(MAX)")
            .Not.Nullable();
        Map(x => x.SiteKeyCaption)
            .Length(128)
            .Not.Nullable();
        Map(x => x.UserMustUpdatePwd)
            .Not.Nullable();

        References(x => x.Account)
            .Class(typeof(Account))
            .Not.Nullable()
            .Column("AccountId"); 
        References(x => x.UserType)
            .Class(typeof(UserType))
            .Not.Nullable()
            .Column("UserTypeID");

        HasMany(x => x.CaseUsers)
            .KeyColumn("UserId")
            .Access.CamelCaseField(Prefix.None)
            .Cascade.All()
            .AsSet();

        HasManyToMany(x => x.Groups)
            .Table("UserGroup")
            .Access.CamelCaseField(Prefix.None)
            .ParentKeyColumn("UserID")
            .ChildKeyColumn("GroupID")
            .Cascade.All()
            .AsBag(); 
    }
}