Casting from class to interface in Excel VBA - vba

Question

In Excel 2013, I have two classes: LoadCase and LoadCombination, which implement interface ILoadCase.
The declaration for ILoadCase is:
Option Explicit
'' Public properties
Public Property Get Name() As String
End Property
Public Property Let Name(ByVal value As String)
End Property
Public Property Get ID() As Long
End Property
Public Property Let ID(ByVal valus As Long)
End Property
And the (partial) implementations for both LoadCase and LoadCombination are:
Option Explicit
Implements ILoadCase
'' Public properties
Public Property Get ILoadCase_Name() As String
ILoadCase_Name = pName
End Property
Private Property Let ILoadCase_Name(ByVal value As String)
pName = value
End Property
Public Property Get ILoadCase_ID() As Long
ILoadCase_ID = pID
End Property
Private Property Let ILoadCase_ID(ByVal value As Long)
pID = value
End Property
I've omitted code which is irrelevant to the implementation of the interface.
I then have a class BeamForces, which contains results for a particular ILoadCase object:
Option Explicit
Public Fx As Double
Public Fy As Double
Public Fz As Double
Public Mx As Double
Public My As Double
Public Mz As Double
Public ParentLoadCase As ILoadCase
I thought that with this I'd be able to do something like this:
Set currentBeamForces = New BeamForces
With currentBeamForces
.Fx = forces(0)
.Fy = forces(1)
.Fz = forces(2)
.Mx = forces(3)
.My = forces(4)
.Mz = forces(5)
Set .ParentLoadCase = TargetLoadCase
End With
Where TargetLoadCase is either a LoadCase or a LoadCombination, but this gives me an error every time.
I've coded this like I would in .NET and just expected that it would work, but does casting to an interface not work in VBA? Or am I going wrong here somewhere?
EDIT
More details. I first call the following method:
Public Function LoadBeamForcesAtNode(ByVal TargetBeam As Beam, ByVal TargetNode As Node, Optional ByVal TargetLoadCases As Collection = Nothing) As Boolean
Dim i As Integer
Dim currentLoadCase As Variant
Dim targetBeamForces As BeamForces
If TargetLoadCases Is Nothing Then
For Each currentLoadCase In Me.LoadCases.Items
Call TargetLoadCases.Add(currentLoadCase)
Next
For Each currentLoadCase In Me.LoadCombinations.Items
Call TargetLoadCases.Add(currentLoadCase)
Next
End If
'On Error GoTo ExitPoint
For Each currentLoadCase In TargetLoadCases
Set targetBeamForces = InstantiateBeamForces(TargetBeam, TargetNode, currentLoadCase)
If TargetNode Is TargetBeam.Node1 Then
Set TargetBeam.Forces1 = targetBeamForces
Else
Set TargetBeam.Forces2 = targetBeamForces
End If
Next
LoadBeamForcesAtNode = True
ExitPoint:
End Function
Where TargetLoadCases is a collection which can contain both LoadCase and LoadCombination objects.
The problem occurs in InstantiateBeamForces, the code for which is
Private Function InstantiateBeamForces(ByVal TargetBeam As Beam, ByVal TargetNode As Node, ByVal TargetLoadCase As Variant) As BeamForces
Dim forces(5) As Double
Dim currentBeamForces As BeamForces
Call Me.output.GetMemberEndForces(TargetBeam.ID, IIf(TargetNode Is TargetBeam.Node1, 0, 1), TargetLoadCase.ILoadCase_ID, forces, 0)
Set currentBeamForces = New BeamForces
With currentBeamForces
.Fx = forces(0)
.Fy = forces(1)
.Fz = forces(2)
.Mx = forces(3)
.My = forces(4)
.Mz = forces(5)
Set .ParentLoadCase = TargetLoadCase
End With
Set InstantiateBeamForces = currentBeamForces
End Function
Which creates a new BeamForces object and populates it with the values returned by the ...GetMemberEndForces(...) API COM call.
The problem is that the .ParentLoadCase property is nothing after the assignment, so I'm assuming an invalid cast...
** EDIT 2 **
Here is a screenshot of TargetLoadCase when I put a breakpoint in InstantiateBeamForces.
The ILoadCase member is Nothing, but I don't get why. Could this be the cause of the problem?