Adding to a collection that is in a class

I'm writing a quoting application in Excel. It's a multilevel deal ... meaning, components get grouped into assemblies, that get grouped into assemblies, that ... repeat up all the levels.

I've written my own class called clsComponents. For each component, I have a property of SubComponents as type Collection. My intention is to fill that collection with clsComponents that are a sub-component of that particular component (are you still with me?).

 I've written a routine that loops through and puts all the subcomponents in an array. It works. However, when I go and loop through the array to assign each sub component to the component, it get an error of ...

Object variable or With block variable not set

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Ok ... now for some code.

This is my routine to assign parent and subcomponent. Ignore the parent stuff as it works.

    For x = 1 To UBound(aryIBOM)
        'CREATE COMPONENT RECORD FOR COMPONENT X
        Set recComponent = colAssembly.Item(aryIBOM(x, 4))

        'FIND THE PARENT COMPONENT AND SET IT FOR COMPONENT X
        colAssembly.Item(aryIBOM(x, 4)).Parent = FindParent(x, aryIBOM)
        'FIND ALL THE SUBCOMPONENTS OF X AND PLACE THEM IN AN ARRAY
        childComponents = FindChildren(x, aryIBOM)
        'LOOP THOUGH AND ASSIGN SUBCOMPONENTS TO THE X
        For y = 1 To UBound(childComponents)
            'CREATE SUBCOMPONENT Y
            Set subComponent = colAssembly.Item(childComponents(y))
            'ASSIGN SUBCOMPONENT Y TO COMPONENT X
            recComponent.AddSubComponent subComponent
        Next y
    Next x

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This is the relevant parts of my class

Public Line As Integer                  'LINE COLUMN FROM INPUT BOM
Public LV As Integer                    'LV COLUMN FROM INPUT BOM
Public PartNumber As String             'PART# FROM INPUT BOM
Public Rev As String                    'REV FROM INPUT BOM
Public Description As String            'DESCRIPTION FROM INPUT BOM
Public Qty As Integer                   'QTY FROM INPUT BOM
Public Parent As String                 'THE COMPONENT ONE LEVEL ABOVE THIS ONE
Public SubComponents As Collection      'COLLECTION CONTAINING REFERENCE TO ALL SUBCOMPONENTS

'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
' NAME:     AddSubComponent
' AUTHOR:   QHOLMBERG
' PURPOSE:
' The class keeps track of all the subcomponents off the assembly
'   through a collection. This function adds a subcomponent to
'   that collection.
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
Public Function AddSubComponent(subComponent As clsComponent)
    Me.SubComponents.Add subComponent, subComponent.PartNumber
End Function

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The error is a result of that line in AddSubComponent function but I'm at a loss as to why. Collections and classes are a little new to me.

Thanks.