Adding a reference to .net class library from ASP.net core web application

I have added a reference to a class library project in the same project as my asp.net core web application. It shows up under Dependencies/Projects

However when I try to refer to any namespace/class inside this referenced project from a asp.net core controller action it just says "the type or namespace name XXX could not be found..."

The refernced DLL/project is full .net framework, not core.