Unable to change an ESXi 5.x server’s hostname in the vSphere client because the OK button is grayed out

I’ve recently been asked at least 5 times over the past 2 months a problem that may seem trivial but at the same time be quite the annoyance.  An administrator attempts to change the hostname of a newly deployed ESXi 5.x server via the vSphere client:

… but notices that as soon as they change the name the OK button grays out:

Editing the IP address doesn’t enable the button either.

The answer to this issue is actually quite simple and that is because the Domain field isn’t filled out and while you can change the hostname at the console without entering a domain, the same can’t be done in the vSphere client so to enable the OK button, simply enter a domain:

Definitely one of the more silly issues I’ve heard about over the past few months.