HLK "Create Package" signing step fails?

Question

I've tried signing with a .cer file (my preferred option), but also via the Certificate Store.  Via both mechanisms the final signing of the package fails. In event Viewer I see:

-

System

- Provider [ Name] HLK (Studio UI)

- EventID 0 [ Qualifiers] 0

Level 2

Task 0

Keywords 0x80000000000000

- TimeCreated [ SystemTime] 2015-06-25T15:32:57.000000000Z

EventRecordID 36347

Channel HLK - Hardware Lab Kit

Computer <computername is here>

Security

-

EventData

Could not create submission package - Signing key is not loaded.

I have code-signing certificate, it is still SHA1, and IS NOT EV -- but an EV one is not required yet, is it?

When I select "Creat Package" I do get the pop-up that says "Signing Options". I select "Use the certificate store", click OK and I get a "Windows Security" pop-up displaying my certificate, Valid From dates, etc.  I select OK. It asks me where to save the .hlkx file, it goes through its 181 steps of "Creating Package" ... and then I get the pop-up "HLK Error Message: Unable to use the selected certificate to sign the package", and the Event Log event shown above.

Where else can I look to try and gather more information or resolve this?  Are there special attributes of the signing Certificate that I've missed?

Thanks for any help or pointers,