ssh job fails when command is executed successfully

using private key with passphrase.

command has variable substitutions:

ln sfn /home/bamboo/puppet/${bamboo.ReleaseDirectoryPrefix}${bamboo.repository.revision.number} /home/bamboo/puppet/current

command is successfully executed on target host, but job fails. [exit code = null]

----------

LOG FILE RESULTS:

04-Nov-2012 14:15:54 Starting task 'create link to current release' of type 'com.atlassian.bamboo.plugins.bamboo-scp-plugin:sshtask'
04-Nov-2012 14:15:55 Executing 'ln -sfn /home/bamboo/puppet/puppet-prod-de086fd971fdbde80aa3a3cb74aa4a45f4001e1a /home/bamboo/puppet/current'
04-Nov-2012 14:15:55 Command: ln -sfn /home/bamboo/puppet/puppet-prod-de086fd971fdbde80aa3a3cb74aa4a45f4001e1a /home/bamboo/puppet/current
04-Nov-2012 14:15:55 Command: stdout =
04-Nov-2012 14:15:55 Command: stderr =
04-Nov-2012 14:15:55 Command: exit = null
04-Nov-2012 14:15:55 'ln -sfn /home/bamboo/puppet/puppet-prod-de086fd971fdbde80aa3a3cb74aa4a45f4001e1a /home/bamboo/puppet/current' was run.
04-Nov-2012 14:15:55 Error occurred while running Task 'create link to current release(4)'. This build will fail. Caused by:
04-Nov-2012 14:15:55 java.lang.NullPointerException
04-Nov-2012 14:15:55 at com.atlassian.bamboo.plugins.scp.SshTask.execute(SshTask.java:104)
04-Nov-2012 14:15:55 at com.atlassian.bamboo.task.TaskExecutorImpl.executeTasks(TaskExecutorImpl.java:180)
04-Nov-2012 14:15:55 at com.atlassian.bamboo.task.TaskExecutorImpl.execute(TaskExecutorImpl.java:93)
04-Nov-2012 14:15:55 at com.atlassian.bamboo.build.pipeline.tasks.ExecuteBuildTask.call(ExecuteBuildTask.java:87)
04-Nov-2012 14:15:55 at com.atlassian.bamboo.v2.build.agent.DefaultBuildAgent.build(DefaultBuildAgent.java:206)
04-Nov-2012 14:15:55 at com.atlassian.bamboo.v2.build.agent.BuildAgentControllerImpl.waitAndPerformBuild(BuildAgentControllerImpl.java:103)
04-Nov-2012 14:15:55 at com.atlassian.bamboo.v2.build.agent.DefaultBuildAgent$1.run(DefaultBuildAgent.java:111)
04-Nov-2012 14:15:55 at com.atlassian.bamboo.build.pipeline.concurrent.NamedThreadFactory$2.run(NamedThreadFactory.java:52)
04-Nov-2012 14:15:55 at java.lang.Thread.run(Thread.java:662)