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
I have VS 2013. According to the extensions manager, nuget was up to date.
Ran a manual install from the vsix site: http://visualstudiogallery.msdn.microsoft.com/4ec1526c-4a8c-4a84-b702-b21a8f5293ca
... fixed the issue.