Which specific version of MySql 5.7 were you using? Based on a little research I've done, it appears that this may be a MySql problem that was resolved in 5.7.7 for the InnoDB engine. If you are using 5.7.7 or later with the InnoDB engine, then you may be able to resolve the issue by setting innodb_large_prefix=on and ROW_FORMAT=DYNAMIC.