Problem with adding a custom attribute to a workItem

Maybe someone can help me with this problem.
I just want to add a custom attribute, which doesn't work for some reason.

public void addAttributes(IWorkItem workItem, List<IAttribute> attList) {
try {
IWorkItem wi = copyManager.getWorkingCopy(workItem);

for (IAttribute elem : attList) {
if (!wi.hasCustomAttribute(elem)){
wi.addCustomAttribute(elem); // ASSERTION FAILED EXCEPTION
}
}
} catch (TeamRepositoryException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}

}


org.eclipse.core.runtime.AssertionFailedException: assertion failed:
at org.eclipse.core.runtime.Assert.isTrue(Assert.java:109)
at org.eclipse.core.runtime.Assert.isTrue(Assert.java:95)
at com.ibm.team.workitem.common.internal.model.impl.WorkItemImpl.addCustomAttribute(WorkItemImpl.java:2796)

Thanks in advance.