Certain mesh will not export...

I have created a non-manifold mesh that I wanted to export as an obj to another program.

The issue is that it does not seem to export anything - even my usual savior meshlab throws an error.

Helmet.blend (1.36 MB)

I’ve never had a problem before so I’ve obviously done something to the mesh.

Can anyone tell me what?

Many thanks in advance.

Joe.