SELECT UNION with JOIN results in class cast exception

Is possible to do a query with JOIN and UNION ? Don't know if is the desired behavior but execution of query:

SELECT a.* FROM [my:node] AS b JOIN [my:node] as a ON b.[ref1] = a.[jcr:uuid] UNION SELECT a.* FROM [my:node] AS b JOIN [my:node] as a ON b.[ref2] = a.[jcr:uuid]

Result:

java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to org.modeshape.jcr.query.QueryResults$Location
at org.modeshape.jcr.query.process.ProcessingComponent$20.compare(ProcessingComponent.java:515)
at org.modeshape.jcr.query.process.ProcessingComponent$20.compare(ProcessingComponent.java:511)
at java.util.Arrays.mergeSort(Arrays.java:1270)
at java.util.Arrays.mergeSort(Arrays.java:1281)
at java.util.Arrays.sort(Arrays.java:1210)
at java.util.Collections.sort(Collections.java:157)
at org.modeshape.jcr.query.process.SortLocationsComponent.execute(SortLocationsComponent.java:52)
at org.modeshape.jcr.query.process.UnionComponent.execute(UnionComponent.java:48)
at org.modeshape.jcr.query.process.QueryProcessor.execute(QueryProcessor.java:96)
at org.modeshape.jcr.query.process.QueryEngine.execute(QueryEngine.java:140)
at org.modeshape.jcr.query.lucene.LuceneQueryEngine$1.getResults(LuceneQueryEngine.java:151)
at org.modeshape.jcr.query.JcrQuery.execute(JcrQuery.java:119)

(At the specified index position in the tuple there is a collection of uuid strings.)