Shadow file entry question

[Chandler, Scott]

We've got a system which is being audited for SOX compliance. The auditors are asking about entries in the shadow file. For the users haldaemon and messagebus, the password section contains a single x. Note, this is the shadow file, not the passwd file. Any idea what that x denotes?

haldaemon:x:14517:0:99999:7:::0
messagebus:x:14517:0:99999:7:::0

Thanks,
Scott