Mt could not find index server


After doing an online upgrade of our couchbase 3.0.1 community edition cluster to couchbase 4.0.0 we are getting the following message when performing a backup of our cluster:

2015-10-10 07:10:14,161: mt could not find index server:0

The cluster was upgraded by adding 2 couchbase 4.0.0 servers and removing the old 3.0.1 servers one by one. How can we resolve this message? Or is there nothing to worry about? The backup is performed using the cli: /opt/couchbase/bin/cbbackup http://localhost /opt/backup -u -p -m full

Could anyone please elaborate on this?