AutoCAD .NET :: Program Crashes When Trying To Access Modelspace

I have a program where I am inserting blocks at specific points in model space.  I have successfully been able to create a subroutine that will accept a block name, a point3D Location and a layer name and and insert the specified block at the specified location on the specified layer.  What I need to do now is to cycle through model space and find all of the pipe objects in model space.  I then find the endpoint of each pipe and insert a block at that location.  The problem that I am having is that the program crashes when I try to find the objectid of modelspace.  I am using the technique discussed in this thread by Jeffrey_H.

VB.NET Access to Styles

Below is the code that I am trying to get to work.
 
'Test Command to insert the some blocks into the drawing <CommandMethod("TrimbleTestBlocks")> _ Public Sub TrimbleTestBlocks() 'Get the editor object Dim doc = AcApp.DocumentManager.MdiActiveDocument Dim db =

[Code]....
 
This is the line that crashes on me

 'Get Model Space Block Table Record Dim mdlspace As BlockTableRecord = SymbolUtilityServices.GetBlockModelSpaceId(db).GetObject(OpenMode.ForRead)
 
why this is crashing the program?  I just started programming dotnet this weekend and my head is starting to explode.

My end goal is to cycle through every pipe in model space.  If the pipe belongs to a specified system definition then find the pipe size and insert a corresponding block on the endpoint of the pipe.  I then want to export the blocks to a new drawing to be imported into a trimble unit for locating inserts on a deck.