DROP INDEX CASCADE doesn't want to drop unique constraints?

Hello,

On trying to drop an index named "employer_employerid_key" which supports a
unique constraint:

"employer_employerid_key" UNIQUE CONSTRAINT, btree (employerid)

I get this error:

ERROR: cannot drop index employer_employerid_key because constraint
employer_employerid_key on table employer requires it
HINT: You can drop constraint employer_employerid_key on table employer
instead.

I'm using the CASCADE and IF EXISTS arguments and the docs say nothing
about any special cases (
https://www.postgresql.org/docs/9.3/static/sql-dropindex.html). This is
with PostgreSQL 9.3.15.

The actual command is:

drop index if exists employer_employerid_key cascade;

Any ideas if this is normal or why it happens?