Unable to add ESXi4.1 host to VSphere 5.1 VCenter

Hello,

I have tried adding a ESXi4.1 host to a VCenter 5.1 server and receive the following:

I am of the opinion that the license used on the host is a free edition and this is why I am receiving the above. The licenses installed on the VCenter are Essentials 5.1

Could there be any other possible reason why I am receiving the above before I go back to the customer?

Many thanks

Simon