I have a job A which triggers job B from a build trigger build step. The job B contains a single msbuild step.
The job B builds fine when started independently, but fails when started as a downstream job.
Here is what it outputs:

Path To MSBuild.exe: msbuild.exe
FATAL: null
java.lang.NullPointerException
at hudson.plugins.msbuild.MsBuildBuilder.perform(MsBuildBuilder.java:94)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:692)
at hudson.model.Build$RunnerImpl.build(Build.java:178)
at hudson.model.Build$RunnerImpl.doRun(Build.java:139)
at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:462)
at hudson.model.Run.run(Run.java:1404)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:238)