MySQL: Invalid default value for TIMESTAMP

I get the error

ERROR 1067 (42000) at line 5459: Invalid default value for 'start_time'

when running the following query

DROP TABLE IF EXISTS `slow_log`;
CREATE TABLE IF NOT EXISTS `slow_log` (
  `start_time` timestamp(6) NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `user_host` mediumtext NOT NULL,
  `query_time` time(6) NOT NULL,
  `lock_time` time(6) NOT NULL,
  `rows_sent` int(11) NOT NULL,
  `rows_examined` int(11) NOT NULL,
  `db` varchar(512) NOT NULL,
  `last_insert_id` int(11) NOT NULL,
  `insert_id` int(11) NOT NULL,
  `server_id` int(10) unsigned NOT NULL,
  `sql_text` mediumtext NOT NULL
) ENGINE=CSV DEFAULT CHARSET=utf8 COMMENT='Slow log';

I am using MySQL 5.7.18

$ mysql --version
mysql  Ver 14.14 Distrib 5.7.18, for osx10.10 (x86_64) using  EditLine wrapper

According to the MySQL 5.7 documentation, the following syntax is

CREATE TABLE t1 (
  ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  dt DATETIME DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
);

What is wrong with the SQL syntax above?

Interestingly, both these work:

`start_time` timestamp(6), 

And:

`start_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,

You can use the latter -- leave the precision specifier out of the definition.

But the right method is:

`start_time` timestamp(6) NOT NULL DEFAULT CURRENT_TIMESTAMP(6) ON UPDATE CURRENT_TIMESTAMP(6),

As explained in the documentation:

If a TIMESTAMP or DATETIME column definition includes an explicit fractional seconds precision value anywhere, the same value must be used throughout the column definition. This is permitted:

CREATE TABLE t1 (
  ts TIMESTAMP(6) DEFAULT CURRENT_TIMESTAMP(6) ON UPDATE CURRENT_TIMESTAMP(6)
);

This is not permitted:

CREATE TABLE t1 (
  ts TIMESTAMP(6) DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP(3)
);