Slow Cooker makes a mess

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

I bought a slow cooker (http://ift.tt/1Dpcero) which seemed to have good reviews however I try to make stock which I would normally cook for 12+ hours however 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.