Can't move extension out of pg_catalog

After moving an extension into the pg_catalog schema, it seems
impossible to move it somewhere else:

test=# create extension ltree;
CREATE EXTENSION
test=# alter extension ltree set schema pg_catalog;
ALTER EXTENSION
test=# alter extension ltree set schema public;
ERROR:  cannot remove dependency on schema pg_catalog because it is a
system object.

It is still possible to drop it.

Tested with PostgreSQL 9.1.3.

-- Daniele