params = [:]

params['foo'] = 'bar'

will throw:

java.lang.UnsupportedOperationException
at java.util.AbstractMap.put(AbstractMap.java:203)
at groovy.lang.MetaClassImpl.setProperty(MetaClassImpl.java:2713)
at groovy.lang.MetaClassImpl.setProperty(MetaClassImpl.java:3747)
at org.codehaus.groovy.runtime.InvokerHelper.setProperty(InvokerHelper.java:199)
at org.codehaus.groovy.runtime.DefaultGroovyMethods.putAt(DefaultGroovyMethods.java:272)
at org.codehaus.groovy.runtime.dgm$508.invoke(Unknown Source)
at org.codehaus.groovy.runtime.callsite.PojoMetaMethodSite$PojoMetaMethodSiteNoUnwrapNoCoerce.invoke(PojoMetaMethodSite.java:274)
at org.codehaus.groovy.runtime.callsite.PojoMetaMethodSite.call(PojoMetaMethodSite.java:56)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:133)
at org.kohsuke.groovy.sandbox.impl.Checker$9.call(Checker.java:400)
at org.kohsuke.groovy.sandbox.GroovyInterceptor.onSetArray(GroovyInterceptor.java:124)
at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy.SandboxInterceptor.onSetArray(SandboxInterceptor.java:442)
at org.kohsuke.groovy.sandbox.impl.Checker$9.call(Checker.java:397)
at org.kohsuke.groovy.sandbox.impl.Checker.checkedSetArray(Checker.java:394)
at com.cloudbees.groovy.cps.sandbox.SandboxInvoker.setArray(SandboxInvoker.java:44)
at com.cloudbees.groovy.cps.impl.ArrayAccessBlock.rawSet(ArrayAccessBlock.java:26)
at WorkflowScript.run(WorkflowScript:27)
at __cps.transform__(Native Method)
at com.cloudbees.groovy.cps.impl.PropertyishBlock$ContinuationImpl.set(PropertyishBlock.java:88)
at com.cloudbees.groovy.cps.impl.AssignmentBlock$ContinuationImpl.assignAndDone(AssignmentBlock.java:70)
at sun.reflect.GeneratedMethodAccessor149.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.cloudbees.groovy.cps.impl.ContinuationPtr$ContinuationImpl.receive(ContinuationPtr.java:72)
at com.cloudbees.groovy.cps.impl.ConstantBlock.eval(ConstantBlock.java:21)
at com.cloudbees.groovy.cps.Next.step(Next.java:58)
at com.cloudbees.groovy.cps.Continuable.run0(Continuable.java:154)
at org.jenkinsci.plugins.workflow.cps.SandboxContinuable.access$001(SandboxContinuable.java:18)
at org.jenkinsci.plugins.workflow.cps.SandboxContinuable$1.call(SandboxContinuable.java:33)
at org.jenkinsci.plugins.workflow.cps.SandboxContinuable$1.call(SandboxContinuable.java:30)
at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy.GroovySandbox.runInSandbox(GroovySandbox.java:108)
at org.jenkinsci.plugins.workflow.cps.SandboxContinuable.run0(SandboxContinuable.java:30)
at org.jenkinsci.plugins.workflow.cps.CpsThread.runNextChunk(CpsThread.java:163)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup.run(CpsThreadGroup.java:324)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup.access$100(CpsThreadGroup.java:78)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup$2.call(CpsThreadGroup.java:236)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup$2.call(CpsThreadGroup.java:224)
at org.jenkinsci.plugins.workflow.cps.CpsVmExecutorService$2.call(CpsVmExecutorService.java:63)
at java.util.concurrent.FutureTask.run(FutureTask.java:262)
at hudson.remoting.SingleLaneExecutorService$1.run(SingleLaneExecutorService.java:112)
at jenkins.util.ContextResettingExecutorService$1.run(ContextResettingExecutorService.java:28)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask.run(FutureTask.java:262)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)
Finished: FAILURE

Current whitelist:

method groovy.lang.GroovyObject getProperty java.lang.String
method groovy.lang.GroovyObject invokeMethod java.lang.String java.lang.Object
method java.lang.Throwable getMessage
method java.util.Map size
staticMethod org.codehaus.groovy.runtime.DefaultGroovyMethods print java.lang.Object java.lang.Object
staticMethod org.codehaus.groovy.runtime.DefaultGroovyMethods sprintf java.lang.Object java.lang.String java.lang.Object
staticMethod org.codehaus.groovy.runtime.DefaultGroovyMethods take java.lang.CharSequence int
staticMethod org.codehaus.groovy.runtime.ScriptBytecodeAdapter createRange java.lang.Object java.lang.Object boolean

renaming the variable 'params' will fix this issue.