AutoCAD Civil 3D :: File Contains External References Function Fails

I'm trying to adjust the properties of some block objects in Mspace. When the file contains external references the function fails, when it has none it works perfectly.

Dim modelSpace As BlockTableRecord = DirectCast(M_trans.GetObject(SymbolUtilityServices.GetBlockModelSpaceId(db), OpenMode.ForRead), BlockTableRecord)
                For Each id As ObjectId In modelSpace
                    Dim ent As Autodesk.AutoCAD.DatabaseServices.Entity = DirectCast(M_trans.GetObject(id, OpenMode.ForWrite), Autodesk.AutoCAD.DatabaseServices.Entity)
                    ent.Color = Color.FromColorIndex(ColorMethod.ByLayer, 256)
                  
Using the above to get all objectid's and filter through them then using:

  If TypeOf ent Is BlockReference Then
                            Dim br As BlockReference = DirectCast(ent, BlockReference)
                            Dim btr As BlockTableRecord = DirectCast(M_trans.GetObject(br.BlockTableRecord, OpenMode.ForRead), BlockTableRecord)
                            If Not btr.IsFromExternalReference Then