NullPointerException when triggering dependent builds

Build completes satisfactorily, but NPE while triggering dependent builds. I upgraded to 1.623 to see if that resolved the fault, but the results are identical. The log shows only 4 of 10 builds triggered, but it may not have been flushed at the point of the fault. All builds are freestyle using multiple shell steps, with no Java or Groovy components or steps. This problem is 100% repeatable with certain builds.

Tail of an example failing build:
17:44:08 Archiving artifacts
17:44:11 Warning: you have no plugins providing access control for builds, so falling back to legacy behavior of permitting any downstream builds to be triggered
17:44:11 Triggering a new build of trap
17:44:11 Triggering a new build of snort-e
17:44:11 Triggering a new build of tcparchive_982
17:44:11 Triggering a new build of deepinspect
17:44:11 FATAL: null
17:44:11 java.lang.NullPointerException
17:44:11 at jenkins.triggers.ReverseBuildTrigger.shouldTrigger(ReverseBuildTrigger.java:111)
17:44:11 at jenkins.triggers.ReverseBuildTrigger.access$000(ReverseBuildTrigger.java:84)
17:44:11 at jenkins.triggers.ReverseBuildTrigger$1.shouldTriggerBuild(ReverseBuildTrigger.java:141)
17:44:11 at hudson.tasks.BuildTrigger.execute(BuildTrigger.java:245)
17:44:11 at hudson.model.AbstractBuild$AbstractBuildExecution.cleanUp(AbstractBuild.java:687)
17:44:11 at hudson.model.Build$BuildExecution.cleanUp(Build.java:200)
17:44:11 at hudson.model.Run.execute(Run.java:1788)
17:44:11 at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
17:44:11 at hudson.model.ResourceController.execute(ResourceController.java:98)
17:44:11 at hudson.model.Executor.run(Executor.java:381)
17:44:11 Finished: SUCCESS