Outlook Ribbon Load Inspector.CurrentItem is null

Outlook Ribbon Load Inspector.CurrentItem is null

Overview
I have an Outlook add-in created with VSTO. The add-in has a single ribbon
(visual designer) for the Mail.Compose ribbon type. The ribbon tab
ControlIdType is set to "Custom". The only code in the add-in other than
designer code is the following Load handler for the ribbon.
this.Context.CurrentItem is unexpectedly returning null.
Code
private void RibbonComposeMail_Load(object sender, RibbonUIEventArgs e)
{
try
{
var inspector = this.Context as Outlook.Inspector;
if (inspector == null)
{
throw new ApplicationException("Fail - Step 1");
}
var currentMailItem = inspector.CurrentItem as Outlook.MailItem;
if (currentMailItem == null)
{
throw new ApplicationException("Fail - Step 2");
}
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
}
}
Steps
Open draft email. Ribbon loads OK.
Open email from inbox.
Open same draft email. Ribbon fails at step 2, inspector.CurrentItem is null.
Notes
I've tested this in Outlook 2007, 2010 and 2013, with an Outlook 2007 and
2010 add-in created in VS2010, and Outlook 2010 add-in created in VS2012.
All behave the same.
Repeatedly opening the draft email doesn't appear to cause the issue, an
Email.Read inspector has to be opened in between.
The ribbon tab ControlidType matters. "Custom" will cause the issue, but
the default option of "Office" doesn't exhibit the issue.
Flipping the scenario on its head and setting the ribbon type to Mail.Read
gives the same result, provided the sequence of opening is reversed to
Inbox > Draft > Inbox (fail).
All possible permutations of calls to Marshal.ReleaseComObject on the
inspector and currentMailItem objects makes no difference.