Undefined index in Client.php

[awesomeousPrime]

We are getting the following error for one customer so far :

ErrorException
Undefined index: 31ec... (GUID)

In the following lines starting at src/Evernote/Client.php:315

foreach ($linkedNotebooks as $linkedNotebook) {
    if (array_key_exists($linkedNotebook->shareKey, $sharedBusinessNotebooks)) {
        $sharedNotebook = $sharedBusinessNotebooks[$linkedNotebook->shareKey];
        $businessNotebook = $businessNotebooksGuids[$sharedNotebook->notebookGuid];
...

It looks like a notebook was found in linkedNotebooks that was also available in sharedBusinessNotebooks but was not found in businessNotebooksGuids which comes from :

$businessNotebooks = $this->getBusinessLinkedNotebooks();

foreach ($businessNotebooks as $businessNotebook) {
    $businessNotebooksGuids[$businessNotebook->guid] = $businessNotebook;
}

I'm not certain of the intent and am wondering if the problem is that the linked notebook wasn't in the business link notebook array or if expecting it to always be there is the issue.