DROP named primary index through UI is not working

Don't know it is UI or query or secondary index issue.

Repro:

CREATE bucket b1;
CREATE scope b1.s1;
CREATE collection b1.s1.c1;
CREATE PRIMARY INDEX ix1 ON b1.s1.c1;

Go to UI----->Indexes---->pick ix1 === > Drop  == errors

N1QL getting following statement (DROP INDEX `b1`.`ix1`) ix1 is created on the collection.

QWB: DROP INDEX ix1 ON b1.s1.c1; – works.

https://forums.couchbase.com/t/possible-bug-can-not-drop-primary-index-in-couchbase-7-0-beta/28491/2