Ascend button appearing before clearing boss

[Tremir]

I'm after my 1st ascension, and today I got to the final boss on Eta, and the "you can ascend" button appeared as soon as I cleared zone 9 and entered the boss zone for the first time.
I clicked it, and it brings me to the confirmation screen.
I didn't try to actually ascend though, so no idea if it actually lets me.

[Ray]

Yes, the boss of the last available zone is optional. Once youve 'reached' the zone, you can ascend.