Extension Activation

After a handful of installs, uninstalls and a current 10.6_1 (revision 1 on 10.6) on El Capitan (OS X), I can’t seem to
getone extension working: 

mydb=# CREATE EXTENSION IF NOT EXISTS htore;
ERROR:  could not open extension control file
"/usr/local/Cellar/postgresql@10/10.6_1/share/postgresql@10/extension/htore.control":No such file or directory 

However, it does exist:

$ /usr/local/Cellar/postgresql@10/10.6_1/share/postgresql@10/extension] ll hstore.control
-rw-r--r--  1 rich  admin   158B Feb  6 23:17 hstore.control

I don’t think this is a homebrew error (the package manager I used to install PostgreSQL), but I’ve informed them.

Is there anything that anybody sees here that I’m doing wrong?  I’ve tried single quotations and double quotations
around‘hstore’ and I get the same error.  Other extensions install properly.   

Any advice appreciated.