Esxi5.1 with server 2008r2 key error?

This may be the wrong place to ask this but just want to make sure that Esxi doesnt cause this.

I have ESXI 5.1 essentials as my host with windows 2008r2 as a guest I am trying to enter the windows server key to activate but it gives me this error

Invalid Product key the key you entered doesnt seem to be a valid windows server  key??

I bought the software and the key from the same place downloaded and install the os the key is a MAK key

So does this have anything to do with esxi and some configuration?

Regards