The method JMXNamespaces.narrowToNamespace(MBeanServerConnection, String) throws IllegalArgumentException if either argument is null (though that is not yet documented), OR if the namespace does not exist. These are two fundamentally different things and it is unhelpful for the same exception to cover both of them. We should define a new NoSuchNamespaceException for this and similar cases (notably ClientContext.withContext). It should be a RuntimeException.