Missing private address property from unit.info

When I juju ssh to tomcat/1 I was able to find a command that gives the private address:
ubuntu@juju-hp-mbruzek-machine-5:~$ juju-run tomcat/1 "unit-get private-address"
10.4.189.86

However the equivalent command in Amulet does not produce the same output:
addr, rc = d.sentry.unit['tomcat/1'].run('juju-run tomcat/1 "unit-get private-address"')

Just to follow up, I did find a way to work around this problem from the amulet test perspective. I wanted to leave this information here in case it helps someone else. Warning, the workaround is ugly python code but it works.

# The command must contain the double quotes around "unit-get private address"
# Other ways of writing the command, got the quotes escaped.
cmd = 'juju run --unit tomcat/1 "unit-get private-address"
process = subprocess.Popen(cmd, stdout=subprocess.PIPE, shell=True)

FWIW, a function like the following seems to work:

def get_private_address(unit):
    """Return the private address of the given sentry unit."""
    address, retcode = unit.run('unit-get private-address')
    assert retcode == 0, 'error retrieving unit private address'
    return address