Comment 3 for bug 1333682

The plot thickens.
I edited agent.conf to remove that line, and restarted the 1.18.1 agent, which now is panicing:
2014-06-24 13:08:53 INFO juju.cmd supercommand.go:297 running juju-1.18.1.1-trusty-amd64 [gc]
2014-06-24 13:08:53 INFO juju.cmd.jujud machine.go:127 machine agent machine-0 start (1.18.1.1-trusty-amd64 [gc])
2014-06-24 13:08:53 DEBUG juju.agent agent.go:384 read agent config, format "1.18"
2014-06-24 13:08:53 INFO juju.cmd.jujud machine.go:155 Starting StateWorker for machine-0
2014-06-24 13:08:53 INFO juju runner.go:262 worker: start "state"
2014-06-24 13:08:53 INFO juju.state open.go:81 opening state; mongo addresses: ["localhost:37017"]; entity "machine-0"
2014-06-24 13:08:53 INFO juju runner.go:262 worker: start "api"
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0x46686b]

goroutine 9 [running]:
runtime.panic(0xcdad60, 0x1a86648)
        /usr/lib/go/src/pkg/runtime/panic.c:266 +0xb6
launchpad.net/juju-core/agent.(*configInternal).OpenAPI(0xc2100da000, 0x0, 0x0, 0x0, 0x0, ...)
        /build/buildd/juju-core-1.18.1/src/launchpad.net/juju-core/agent/agent.go:589 +0x3db
main.openAPIState(0x7f1326061cb8, 0xc2100da000, 0x7f1326061b88, 0xc21004fb40, 0xc2100da000, ...)
        /build/buildd/juju-core-1.18.1/src/launchpad.net/juju-core/cmd/jujud/agent.go:182 +0x55
main.(*MachineAgent).APIWorker(0xc21004fb40, 0xc210043d60, 0x10, 0x7f1325ee4ec8, 0x1, ...)
        /build/buildd/juju-core-1.18.1/src/launchpad.net/juju-core/cmd/jujud/machine.go:181 +0x10b
main.func·006(0xeb72d0, 0x10, 0x7f1325ee4ec8, 0x1)
        /build/buildd/juju-core-1.18.1/src/launchpad.net/juju-core/cmd/jujud/machine.go:159 +0x35
launchpad.net/juju-core/worker.(*runner).runWorker(0xc210037d80, 0x0, 0xe0cb10, 0x3, 0xc210073be0)
        /build/buildd/juju-core-1.18.1/src/launchpad.net/juju-core/worker/runner.go:263 +0x306
created by launchpad.net/juju-core/worker.(*runner).run
        /build/buildd/juju-core-1.18.1/src/launchpad.net/juju-core/worker/runner.go:179 +0x356

So potentially it is a different bug, which is that 1.19 is actually *removing* the line that used to be there.