Error registering hashmap with nested hashset collection

[Tony Park]

I'm trying to create a field in an entity that is a hashmap and one of the values is a hashset like this:

private HashMap<Long, HashSet<Long>> hashmap = new HashMap<Long, HashSet<Long>>();

But I get this error when initializing objectify. 
java.lang.IllegalStateException: At path 'hashmap': Error registering at com.googlecode.objectify.impl.Path.throwIllegalState(Path.java:107)

In the objectify documentation it seems that this should be possible: https://github.com/objectify/objectify/wiki/Entities#maps

Am I doing something wrong?