We are trying to migrate from master only to master/slave architecture. We have Linux based master and two Linux slaves so far. Some of the builds are failing on slave with the NPE below. When simply rerun on the master - they work fine.
Not all the projects are failing that are built on the slave. Most are working fine. I see no differences repository state wise between the ones that failed and ones that built successfully on the same slave node. This exception doesn't get logged into the log file, there is no information about the failed build as well other than the job log.

the jobs are set to build qa and master branches, before that we merge onto origin/master to check if the qa branch changes are fast-forwardable (no conflicts) thus the "Merging revision [SHA-1] onto ...". But in the error below it tries to merge onto null.

jenkins 1.436, git plugin 1.11

we will try to update plugin and see if the problem persists

Exception:

Seen branch in repository origin/HEAD
Seen branch in repository origin/master
Seen branch in repository origin/qa
Commencing build of Revision 16b1541a246e350453da70d469c592c2fe2406ce (origin/qa)
Merging Revision 16b1541a246e350453da70d469c592c2fe2406ce (origin/qa) onto null
FATAL: null
java.lang.NullPointerException
at hudson.plugins.git.GitAPI.revParse(GitAPI.java:243)
at hudson.plugins.git.GitSCM$3.invoke(GitSCM.java:1075)
at hudson.plugins.git.GitSCM$3.invoke(GitSCM.java:1058)
at hudson.FilePath$FileCallableWrapper.call(FilePath.java:2022)
at hudson.remoting.UserRequest.perform(UserRequest.java:118)
at hudson.remoting.UserRequest.perform(UserRequest.java:48)
at hudson.remoting.Request$2.run(Request.java:287)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)