Cortana - can't play song/artist because streaming is not enabled in the music app. What's causing this?

[Windows Central Question]

Cortana - can't play song/artist because streaming "is not enabled in the music app"

I can stream music inside the Groove/Xbox Music app just fine, on my Lumia 925 running WP 8.1 by searching/typing. However, if I perform a Cortana voice command to "play [Artist]" - where [Artist] is not already downloaded on my phone - I get a voice response from Cortana that she cannot play it because "streaming is not enabled in the music app."

I've searched high and low through what appear to be the app settings (which are very minimal), and I can't find such a thing. What's tripping Cortana up?

Thanks.