Slow Cooker makes a mess

I tried googling this one but I just kept getting recipes.

I bought a slow cooker (Proctor-Silex 33043 4-Quart Slow Cooker), which seemed to have good reviews. However I try to make stock, which I would normally cook for 12+ hours, every time that I do it bubbles over. It isn't to say that the liquid itself boils up and out of the pot but rather a ring of liquid forms on the edge of the pot and begins to bubble outwards.

It causes a mess by burning inside of the metal part of the pan (in-between the heating element and the stoneware part) and spilling down the outside walls of the slow cooker onto the counter.

What can I do about this? Is it just a bad slow cooker or is there a trick I'm missing. I haven't encountered this problem before with my old cooker or pot on the stove. I was considering inserting a toothpick in the edge or something.

Has anyone else encountered this? Is it because I'm making very liquid stock vs say a heavier dish like oatmeal? I've tried shallow (less liquid) and very full but it occurs in both scenarios.