API with Key Headers required

Apologies if some of the lingo in the post is off- I'm still very new to API's. I'm trying to use Mashape to get some cool API's into my game, but they require you to send a Key to them using a header, not just in a URL as I have been doing before. I thought I figured out how to send the key but it's still not working. Any ideas??

 Dictionary<string, string> KV = new Dictionary<string, string> ();
 KV.Add ("X-Mashable-Key", key);
 WWW a = new WWW ("https://numbersapi.p.mashape.com/529/math", null, KV);
 yield return a;
 Debug.Log (a.text);