objOpenSTAAD.Property.AssignBeamProperty Error

Hello all,

I'm trying to assign material to the beams in STAAD through VBA using the OpenSTAAD library.  However, when I try to assign the material to the beam through objOpenSTAAD.Property.AssignBeamProperty 7, 1  

I get the a Run Time error in Excel: "Run-time error '-214717851(80010105)   Automation error The server threw an exception"

Does anyone know why this error would come about? I'm new to the OpenSTAAD library, so if anyone could shed insight on this problem, that'll be great. 

My code is as follows: 

BusType= "4.00PipeX40"

objOpenSTAAD.Property.CreatePipePropertyFromTable 1, BusType, 0, 0, 0
objOpenSTAAD.Property.SetMaterialID 1
For b = 0 To Sheet16.Range("E11")
j = Cells(b + 14, 6).Value
k = Cells(b + 14, 7).Value
objOpenSTAAD.Geometry.AddBeam j, k

'assign bus material
If Abs(k - j) > 1 Then
objOpenSTAAD.Property.AssignBeamProperty 7, 1
End If
Next b