In Photos: Unable to create a slideshow

I highlight photos in order to create a slideshow. Photos starts but then gives me two error messages. The first is:

"An error occurred while downloading one or more photos for the slideshow. Please try again later."

The second is:

"An internet connection is required to download larger versions of these items for creating a slideshow."

But of course I have an internet connection, so this message is particularly wierd.

But my real concern is: why is Photos even going to the internet? All these pictures are on my Mac.

Any thoughts out there?