I have a fairly standard setup with simply POCO classes
public class Project
{
public int ProjectId { get; set; }
public string Name { get; set; }
I had the same issue and it took quite a while to find out the solution. In our case, we created a seperated project to handle the Entities and even if the default project in the Package Manager Console was the one handling the Entities, I need to set this project as the default project in order to make it work.
I hope this will help somebody else.