parent -1

[Uwe Brauer]

Hi

A collaborator of mine, after pulling from the common repo wanted to
commit and obtained:

hg commit -m "Msg"

warning: ignoring unknown working parent fb3007ebc0d2!
created new head

hg log -g

Gives

changeset:   66:1406bbd18e67
tag:         tip
parent:      -1:000000000000
user:        John Foo <john at foo.com>
date:        Tue Nov 29 21:35:40 2016 +0200
summary:     Msg

I told him to move out the modified files, to strip, to pull again and
then try to commit again.

Anybody has an explanation?
What does
parent:      -1:000000000000

Mean?

Uwe Brauer