@Override
    void internalAddProperty(final Property p) {
        if (null == properties) properties = new HashMap<>();

        if(!this.properties.containsKey(p.key())) {
            this.properties.put(p.key(), new ArrayList());
        }
        this.properties.get(p.key()).add(p);
    }
This constructor is used by GraphSON when deserializing and the {@link Host} is not known.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/util/detached/DetachedVertexProperty.java