Can't run apps in snaps from another (classic) snap

It seems impossible to run snap_1 from snap_2 where snap_2 is classic. This seems significant.

alan@hal:~$ snap version
series  16
ubuntu  16.04
kernel  4.10.0-41-generic
alan@hal:~$ snap run zzt
cannot execute snap-update-ns: Permission denied
snap-update-ns failed with code 1: No such file or directory

Steps to reproduce.

snap install cool-retro-term --classic
snap install zzt
snap run cool-retro-term

Then, inside cool-retro-term (which, is very cool).

snap run zzt

Here’s a pretty picture.