"AccessViolationException" on Mesh.Intersect function


I keep getting an AccessViolationException

Attempted to read or write protected memory. This is often an
indication that other memory is corrupt.

when attempting to use the Intersect function on a Mesh object that I
have loaded with vertices / subsets etc myself. The mesh itself
renders fine. I am using VB 2005 Express so I cannot debug unmanaged
code, and I am getting no debug output when running using DirectX in
debug mode.

Any suggestions?