Can't subscribe to shared album, but I'm on a Mac with Photos

[kirkbross]

Someone sent me a shared Photos album and I clicked the like to subscribe, but keep getting.

Shared albums are not available for this device.

To subscribe to [person's] “AlbumName” shared album on your iPhone, iPad, iPod touch, or Mac, open your invitation in the Mail app, and click the Subscribe button in the message.

I'm logged into iCloud in the same browser (Safari) and have Photos open and have Photos turned on in Preferences for iCloud.

I'm on the latest Mac Mini with the latest OS X.

What am I doing wrong?