Get Custom Record by Internal ID

I was hoping the helper method I wrote (printed below) would return a custom object by its unique internal id, but instead it returns an error "Invalid custom record object in query."

Is it impossible to get custom objects using GET (and if so then how do I accomplish this), or is there something off in my code below?

Edit: To explain a little further, the reason I'm doing this is because I have a list of objects from a previously-performed search on another custom object list. I need to get related objects (which are custom objects). If I was in the Entity Framework world I would say "I need to get at a navigation property"