environment variable for host in [http_proxy] section not working?

[Will Fiveash]

I did the following:

1. $ export  MY_PROXY=http://foo.com:80/

2. edit ~/.hgrc:
[http_proxy]
host = $MY_PROXY

3. $ hg incoming
abort: error: node name or service name not known

If I set host like so in .hgrc:

[http_proxy]
host = http://foo.com:80/

then hg incoming works.  I'm running Mercurial Distributed SCM (version 
2.2.1) on Solaris 11u1.  Is this a bug?
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