snap try with classic confinement doesn't work

[Alan Pope 🍺🐧🐱 🦄]

$ snap --version
snap 2.20.1ubuntu1
snapd 2.20.1ubuntu1
series 16
ubuntu 16.04

"snap try --help" suggests the following options:-

[try command options]
          --devmode Put snap in development mode and disable security confinement
          --jailmode Put snap in enforced confinement mode
          --classic Put snap in classic mode and disable security confinement

However, --classic doesn't work:-

$ sudo snap try --classic prime/
error: cannot perform the following tasks:
- Mount snap "demo" (unset) (snap "demo" requires consent to use classic confinement)

[Michael Vogt (mvo)]

https://github.com/snapcore/snapd/pull/2671