svn:externals example

Ive read the book, I've Googled, I've searched high and low, and I have not
been able to find an example that works.

I have two projects, foo and bar that share a common "sql" directory. The
foo project is up and running, I'd like to make it's SQL directory available
as an external directory to the bar project.

/foo
   /txt
   /sql

/bar
   /txt

so what I did was

$ svn co http://svn.example.com/bar bar
$ cd bar
$ svn mkdir sql
$ svn propset svn:externals 'http:/svn.example.com/foo/sql' sql

and all I get is "error parsing svn:externals property on 'sql'

what am I doing wrong? Can someone update the svn book with some examples?

-- 
I am often wrong, but I am never in doubt.