Visual Studio project not showing in Manage NuGet Packages for solution select project window

I wanted to include an old SiteMap project of mine in a new Visual Studio solution for a project I am working on, but ran into a bit of trouble wanting to add NuGet packages to it. I was able to install packages on the solution’s projects, and on the SiteMap project separately, but not by clicking on the solution itself selecting the manage NuGet Packages option.

The reason for this was rather obvious, all the projects in the existing solution had .NET Framework 4.5 as target framework, while the old SiteMap project was still on Framework 4.0.

Visual Studio properties options on a project.

A quick update of the project properties in Visual Studio targeting .NET Framework 4.5 instead, made the project show up in the NuGet Package Manager window immediatly.

The manage NuGet Packages for Solution option in Visual Studio.