Query on null fails

PropertyRestriction<String> restriction = Restriction.on(UserTermKeys.FIRST_NAME).isNull();
List<User> users = crowdClient.searchUsers(restriction, 0, EntityQuery.ALL_RESULTS);

java.lang.NullPointerException
	at com.atlassian.crowd.integration.rest.util.SearchRestrictionEntityTranslator.valueToString(SearchRestrictionEntityTranslator.java:238)
	at com.atlassian.crowd.integration.rest.util.SearchRestrictionEntityTranslator.toPropertyRestrictionEntity(SearchRestrictionEntityTranslator.java:124)
	at com.atlassian.crowd.integration.rest.util.SearchRestrictionEntityTranslator.toSearchRestrictionEntity(SearchRestrictionEntityTranslator.java:182)
	at com.atlassian.crowd.integration.rest.service.RestCrowdClient.searchUsers(RestCrowdClient.java:457)