Can't find lists: Invalid Mailchimp API Key in mod_mailchimp2

Just a quick FAQ for those that get the following error message while using the MailChimp Module.

Can't find lists: Invalid Mailchimp API Key: xxxxxxxxxxxxx . You are accessing the wrong datacenter - your client library may not properly support our datacenter mapping scheme.
Error code: 104

Please make sure you're using the entire API key.  The MailChimp API looks for the Datacenter ID at the end of your API key:

xxxxxxxxxxxxx-us2

If you just use the numeric portion, MailChimp won't recognize your datacenter.

(Developer's note:  I'm so used to double-clicking to select a whole word that I've goofed this up myself!  Just don't tell anyone.)