How do I query User Stories that have Successors in Rally? -

i'm trying query wsapi find user stories successors , user stories predecessors, assumed that, tags, use following query:

(successors != null)

unfortunately, returns following error:

could not read: not read instances of class com.f4tech.slm.domain.userstory

i'm @ loss; ({arrayfield} != null) supported specially tags? if so, there better way in query?

tags bit of one-off special case. following should work filterable collection fields:

empty collections:

(successors.objectid = null)

non-empty collections:

(successors.objectid != null)

WIKI