Why should fire extinguishers containing carbon dioxide not be used on magnesium fires? As the title states, why should $\ce{CO2}$ fire extinguishers containing carbon dioxide not be used on fires with magnesium metal?

I understand that $\ce{CO2}$ extinguishers work by displacing oxygen, a crucial component of a fire. Does this somehow not work when magnesium is involved, or is there another reason?
The reaction of magnesium metal (Mg) with carbon dioxide is extremely exothermic (heat evolving). In other words, carbon dioxide makes magnesium metal burn. 

$$\ce{2Mg(s) + CO2(g) -&gt; 2MgO(s) + C(s)}$$