Full text search not matching 2 letter word

Im trying to run a full text query on a two letter keyword 'K7'. I have
set ft_min_word_len=2 and restarted the server and if I view the system
vars in Mysql Workbench it shows it is set correctly.

I have then dropped and re-created the index on the descrip column. It
is an InnoDB table so I cannot do repair table.

Im running the following query which I expect to match the following
record but it doesnt. Full text searches for other words match OK.

select * from asset where type ='DOCUMENTS' and (match(descrip) against
('K7' in boolean mode)) ;

| id  | type                   | descrip           | subtype |
intendeduse  | location    | assetfileid |
| 153 | DOCUMENTS | Telephone Kiosk No. 7 K7 Interior promo photo from
field trial.              | PHOTO   | DISPLAY         | STORAGE 
|         152 |

Any ideas why this is not working?