ALTER TABLE DROP CONSTRAINT not working for unique constraints

Create a table 'newtable' with a varchar column 'myfield'.

Add a unique constraint:
ALTER TABLE newtable ADD CONSTRAINT UNIQUE bla (myfield);

Drop the constraint again
ALTER TABLE newtable DROP CONSTRAINT bla RESTRICT;

You get an error 'You have an error in your SQL syntax'.

According to the knowledge base, this should work (it's basically copy-pasted):
https://mariadb.com/kb/en/sql-99/alter-table-statement/#drop-constraint-clause

Workaround: use DROP INDEX instread of DROP CONSTRAINT
ALTER IGNORE TABLE newtable DROP INDEX bla;