MySQL adds broken comment script after each line
This is a bug on MySQL Workbench &gt;=6.3.* reported in these cases #76578 and HERE.

This is what ALTER TABLE do: 


It adds a COMMENT '' on every line of the SQL Script to apply:




I recommended you to downgrade your MySQL Workbench to 6.2.* until they fix this.

Tested in Ubuntu LTS 14.04 and MySQL Workbench 6.3.4.0.