public Object removeAttribute(Object key) throws InvalidSessionException {
        throwImmutableException();
        //we should never ever reach this point due to the exception being thrown.
        throw new InternalError("This code should never execute - please report this as a bug!");
    }
Immediately {@link #throwImmutableException() throws} an <code>InvalidSessionException</code> in all
cases because this proxy is immutable.
core/src/main/java/org/apache/shiro/session/mgt/ImmutableProxiedSession.java