Document.OptimizeResources() blows up when optimising some PDFs

We have found a scenario where the method Aspose.Pdf.Document.OptimizeResources(..) thows a NullReferenceException. The version of Aspose.Pdf is 10.1.0. Please can you explain what is causing this error and how to fix/workaround the problem?

Attached is a sample VS2013 project that recreates the issue.

Thanks

Ben