Mysql modify column not working [closed]

What I wanna do is to update the field comment_author and change the type from tinytext to varchar(100)

For some reason it complains about “Invaild default value for ‘comment_date'”

Why does it complain when I’m not even trying to change it?
The command I’m using is:

 `mysql> Alter TABLE `wp_comments` MODIFY COLUMN `comment_author` VARCHAR(100);` 

Thanks!