I'm seeing the same problem reported in #JENKINS-6084 when trying to start a freestyle job based on ant:

FATAL: null
java.lang.UnsupportedOperationException
at java.util.AbstractMap$SimpleImmutableEntry.setValue(AbstractMap.java:726)
at hudson.EnvVars.resolve(EnvVars.java:156)
at hudson.model.AbstractBuild.getEnvironment(AbstractBuild.java:708)
at hudson.tasks.Ant.perform(Ant.java:134)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:601)
at hudson.model.Build$RunnerImpl.build(Build.java:174)
at hudson.model.Build$RunnerImpl.doRun(Build.java:138)
at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:416)
at hudson.model.Run.run(Run.java:1248)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:129)

From what I can see the Environment Variables are passed through as an ImmutableMap - the subsequent entries that are assembled into the subsequent maps thereby still seem to contain SimpleImmutableEntry

Steps to reproduce (in my environment):

1) Create a new freestyle project
2) Add Build step -> Ant
3) Select Ant runtime (defined in configuration)
4) Leave target empty