IllegalArgumentException that should not happen

We recently stumbled a couple of times over an IllegalArgumentException when calling ServiceObjects.ungetService. In our case we are always in an async process and the service we are calling unget for ist already unregistered. According to the org.osgi.framework.ServiceObjects Spec the Exception will be thrown under the following circumstances:

 @throws IllegalArgumentException If the specified service object is
	 {@code null} or was not provided by a {@code ServiceObjects}
	 object for the associated service.

I can guarantee that both conditions are not meet in our case. It is simply that the service is already unregistered.

If I read the javadoc correctly it should simply do nothing in such a case. I haven't checked with the Spec though.