Created attachment 1342 [details]
Project that reproduces the issue + expected output
Code that works with MS Visual Studio throws NullReferenceException on Mono 2.8.
The issue is probably somewhere in System.IO.Packaging library for Mono.
When I slightly change input (xml content that I'm trying to zip) everything works as expected but current code won't.
I have attached solution that reproduces the issue. Simply run ConsoleApplication project and it will throw an exception. That exception doesn't occur on MS Visual Studio.
For testing we use MonoDevelop environment (virtual machine with Suse Linux with Mono).
Found the same issue while trying to use EPPlus (http://epplus.codeplex.com/) to read an Excel .xlsx file.
It throws a NullReferenceException on Mono and works OK on MS. Debugging EPPlus I arrive to the line where it tries to read the file through System.IO.Packaging.Package.GetPart(WorksheetUri) to a StreamReader.
I'm using MonoDevelop 3.01. with Mono 2.10.8 on Windows.