dept_signature in 1.6 was set to tinytext. My setup of 1.7rc4 also has it as tinytext. Maybe the upgrader missed it, or it didn't get fixed until rc5. (I haven't upgraded my test box to rc5 yet, I should probably do that) Regardless you can:
Open the database in your favorite database editor (MySQL Admin, phpmyadmin, MySQL Workbench, etc). Open the ost_department table.
change the Type for department signature.
Alternatively if you are versed with CLI you could run:
ALTER TABLE `hmis_tickets`.`ost_department` MODIFY COLUMN `dept_signature` TEXT NOT NULL;