Variable `$__done_notification_transient` is used but is not defined

[frazar]

On a clean installation, I'm getting the following error with v1.6.4.

frazar@hostname> sleep 6
Argument is not a number: ''
/etc/fish/conf.d/done.fish (line 261):
                if test "$__done_notification_transient" -eq 1
                   ^
in function '_done_ended'
in event handler: handler for generic event 'fish_prompt'
No summary specified.

I believe it's because, by default, the __done_notification_transient variable is not defined. Indeed, said variable is defined, no error occurs.

frazar@hostname> set -U __done_notification_transient "0"
frazar@hostname> sleep 6
No summary specified.

[VergeDX]

Same issues here, NixOS 21.05.