Adding a deployment scanner fails with NPE

[standalone@localhost:9999 subsystem=deployment-scanner] ./scanner=foo:add(path=/tmp)
{
"outcome" => "failed",
"failure-description" => {"JBAS014671: Failed services" => {"jboss.server.deployment.scanner.foo" => "org.jboss.msc.service.StartException in service jboss.server.deployment.scanner.foo: java.lang.NullPointerException"}},
"rolled-back" => true
}

server console log shows

14:26:24,968 INFO [org.jboss.as] (Controller Boot Thread) JBAS015874: JBoss EAP 6.0.0.Beta1 (AS 7.1.0.Final-redhat-1) started in 5846ms - Started 134 of 205 services (70 services are passive or on-demand)
14:26:56,316 ERROR [org.jboss.msc.service.fail] (MSC service thread 1-4) MSC000001: Failed to start service jboss.server.deployment.scanner.foo: org.jboss.msc.service.StartException in service jboss.server.deployment.scanner.foo: java.lang.NullPointerException
at org.jboss.as.server.deployment.scanner.DeploymentScannerService.start(DeploymentScannerService.java:165)
at org.jboss.msc.service.ServiceControllerImpl$StartTask.startService(ServiceControllerImpl.java:1811) [jboss-msc-1.0.2.GA-redhat-1.jar:1.0.2.GA-redhat-1]
at org.jboss.msc.service.ServiceControllerImpl$StartTask.run(ServiceControllerImpl.java:1746) [jboss-msc-1.0.2.GA-redhat-1.jar:1.0.2.GA-redhat-1]
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886) [classes.jar:1.6.0_29]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908) [classes.jar:1.6.0_29]
at java.lang.Thread.run(Thread.java:680) [classes.jar:1.6.0_29]
Caused by: java.lang.NullPointerException
at org.jboss.as.server.deployment.scanner.FileSystemDeploymentService.establishDeployedContentList(FileSystemDeploymentService.java:289)
at org.jboss.as.server.deployment.scanner.FileSystemDeploymentService.establishDeployedContentList(FileSystemDeploymentService.java:293)
at org.jboss.as.server.deployment.scanner.FileSystemDeploymentService.<init>(FileSystemDeploymentService.java:195)
at org.jboss.as.server.deployment.scanner.DeploymentScannerService.start(DeploymentScannerService.java:151)
... 5 more