Referencing A ComboBox

I have code on a subform that is referencing another form that is still open (not the parent form, but the form which's button openned the form that the subform is on). I'm trying to get the value from a combobox that contains the data that will filter the subform in question. See below.
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Dim strForm As String
Dim strSQL1 As String, strSQL2 As String, strWhere As String
Dim strSelect As String, strFrom As String, strJoin As String

strSelect = "SELECT tblMainData.* "
strFrom = "FROM tblMainData "
strJoin = "INNER JOIN tsubPermissionList ON tblMainData.ResponsibleParty = tsubPermissionList.FullName "
strWhere = "WHERE(((tsubPermissionList.UserID) = [Forms]![frmMainEntry].Form.[cboSelectUser].Column(0)))"
strSQL1 = strSelect & strFrom & strJoin
strSQL2 = strSelect & strFrom & strJoin & strWhere

If IsNull([Forms]![frmMainEntry].Form.[cboSelectUser].Column(0)) Then
' If the combo is Null, use the whole table as the RecordSource.
Me.RecordSource = strSQL1
Else
Me.RecordSource = strSQL2
End If
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I can't resolve the error:
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Runtime error '3085':
Undefined function
'[Forms]![frmMainEntry].Form.[cboSelectUser].Column' in expression.
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Somehow access thinks that the references to the combobox is a function. Can someone help me with this?

Thanks,
PC