@fllicensing,
Since you are using older version of the product which might show such exception or limitation in it. We recommend you to upgrade to latest APIs set (e.g Aspose.Cells for .NET v19.2 (latest)), you might require to update your existing code when upgrading to latest APIs set.
For your information, over the years, we included many enhancements (regarding existing features) with other fixes and supported some new features. We have made the product more robust and feature rich APIs set now. We also recommend you to kindly see and browse Aspose.Cells for .NET Docs for your reference:
http://www.aspose.com/docs/display/cellsnet
If you still find the issue using latest Aspose.Cells version, do share sample code and template file (if any), we will check it soon.
PS. please note, we cannot evaluate issues in older versions, the fixes are based on latest APIs set only. It is quite possible that in older version the feature might not be supported or might have an issue. Such kind of issue might be already fixed in newer versions. So we recommend you to kindly try using latest version of the product.