Thanks for your posting and using Aspose.Cells for .NET.
We are afraid, you are using quite an older version of Aspose.Cells. We just provide support of the versions which are less than 1-year old.
We strongly recommend you to download and use the latest version: Aspose.Cells
for .NET v126.96.36.199.
It should fix your mentioned exception.
Besides, we have lately done some major enhancements and added new features in our product, so you should also download and give a try to latest offline demos of Aspose.Cells which you can find from theses links.
These demos can work with Visual Studio 2005, 2008 or 2010. Please read the readme.txt file before running the demos.