Response from Postsharp.net is not a Valid Nuget v2 Service Response

After installing PostSharp.net on my machine for a project (I did the MSI install) I started getting errors during the package restore that ended up blocking my builds. They looked a lot like this:

Error: FindPackagesById: EntityFramework.Core Response from https://www.postsharp.net/nuget/packages/FindPackagesById()?id=’EntityFramework.Core’?id=) is not a valid NuGet v2 service response.

Now, an important note here: I’m on a machine that’s seen various updates and changes to VS 2015, and this was a version of PostSharp that wasn’t originally built for the RTM version of Visual Studio. So…this may be entirely circumstantial, but it’s what I ran into.

And it wasn’t just on that one package (others would give the same result) and it wasn’t just on one project. I tried to isolate this, but couldn’t find the source. Why was PostSharp getting in the way of my package restore? Even using DNU from the command line, after I explicitly uninstalled it? I started setting compiler variables to block PostSharp on those projects, but that got frustrating quickly, so I resorted to uninstalling everything I could find of it.

After the uninstall, I still was stumped, same errors all over again. With the help of my friend Donald Belcham, I was able to find traces of PostSharp still on my machine, located in the system-wide NuGet package source feed configuration:

Unchecking that box above does the trick.

Might be an edge case if you run into this, but if you do, and this helps, consider buying Don a scotch!

Happy coding.