I have such error in vs 2012: NuGet Package restore failed for project Miscellaneous Files: Value cannot be null or an empty string. Parameter name: root. 0 0
I see
Had the same issue with VS2013 today. After running an update on installed packages (tools -> extensions and updates) (which included nuget), and installing some updates for VS2013 (windows update had some), it has gone away.
So basically, check for those updates, update, and it might solve the issue.