dovecot - Oct 2019 - subscription namespace

The current documentation makes mention of a "special" subscription 
namespace. The example given:

namespace subscriptions {
   subscriptions = yes
   prefix = ""
   list = no
   hidden = yes
}

namespace inbox {
   inbox = yes
   location    subscriptions = no
[...]

results in a startup error as both namespaces have the same prefix. Was 
the intent for the "inbox" namespace to have an explicit
"INBOX/" prefix?

If this is configured for an existing server that previously had no such 
"INBOX/" prefix namespace - will clients need to be reconfigured?

-- 
Daniel