Replication failed between MySQL Versions 5.5 to 5.6 - CREATE USER failed for 'user'@'%'

After I created a new user on MYSQL 5.5 master using:

Operation CREATE USER failed for 'user'@'%'

FLUSH PRIVILEGES;

MYSQL 5.6 slave replication stopped working, gives this error:

Operation CREATE USER failed for 'user'@'%'

I tried on the slave machine to drop and create the user, stop and start slave, but it didn't help.

I guess that's because of changes between mysql versions.

Any idea how to fix this ?