Set group owner using Powershell

I use this command:

Set-SPOSiteGroup -Site https://.....com -Identity newgroup -Owner newgroup2

(newgroup is a Sharepoint group, newgroup2 is another sharepoint group)

Powershell responds with this: Set-SPOSiteGroup: The user newgroup2 was not found.

What is wrong with my command? I am sure newgroup2 exists, and i am able to make it the owner of newgroup using the sharepoint interface.