ImmutablePropertyException while setting target iteration

Hello,

i try to set the target iteration through plain java API. the method.

workItem.setTarget(IIterationHandle i) throws the ImmutablePropertyException. I get the IIterationHandle through this code:

IDevelopmentLine developmentLine = (IDevelopmentLine) getCompleteItem(developmentLineHandle);



for (IIterationHandle iterationHandle : developmentLine

					.getIterations()) {



				IIteration iteration = (IIteration) getCompleteItem(iterationHandle);



                       if (resultIteration != null) {



					return resultIteration;

				}



			}

The Exception is thrown, when I try to set the target.

The stackTrace is:

Exception in thread "main" com.ibm.team.repository.common.internal.ImmutablePropertyException
at com.ibm.team.repository.common.internal.util.ItemUtil$ProtectAdapter.notifyChanged(ItemUtil.java:2060)
at org.eclipse.emf.common.notify.impl.BasicNotifierImpl.eNotify(BasicNotifierImpl.java:280)
at com.ibm.team.workitem.common.internal.model.impl.WorkItemImpl.setTarget(WorkItemImpl.java:3113)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.ibm.team.repository.common.internal.util.ItemStore$ItemInvocationHandler.invoke(ItemStore.java:597)
at $Proxy18.setTarget(Unknown Source)

Does someone know how to get rid of this exeption and set the target?