diff --git a/core/src/main/scala/org/renci/relationgraph/RelationGraph.scala b/core/src/main/scala/org/renci/relationgraph/RelationGraph.scala
index b0a30df..f30009d 100644
--- a/core/src/main/scala/org/renci/relationgraph/RelationGraph.scala
+++ b/core/src/main/scala/org/renci/relationgraph/RelationGraph.scala
@@ -66,11 +66,11 @@ object RelationGraph extends StrictLogging {
       allClasses(ontology).map(c => ZIO.succeed(processSubclasses(c, indexedWhelk.state, outputConfig.reflexiveSubclasses, outputConfig.equivalenceAsSubclass, outputConfig.outputClasses, outputConfig.outputIndividuals)))
     } else ZStream.empty
     val streamZ = for {
-      queue <- Queue.unbounded[Restriction]
+      queue <- Queue.unbounded[Option[Restriction]]
       activeRestrictions <- Ref.make(0)
       seenRefs <- ZIO.foreach(allProperties)(p => Ref.make(Set.empty[AtomicConcept]).map(p -> _)).map(_.toMap)
       _ <- traverse(specifiedProperties, properties, classes, queue, activeRestrictions, seenRefs)
-      restrictionsStream = ZStream.fromQueue(queue).map(r => processRestrictionAndExtendQueue(r, properties, classes, indexedWhelk, outputConfig.mode, specifiedProperties.isEmpty, outputConfig.outputClasses, outputConfig.outputIndividuals, queue, activeRestrictions, seenRefs))
+      restrictionsStream = ZStream.fromQueue(queue).collectWhile { case Some(r) => r }.map(r => processRestrictionAndExtendQueue(r, properties, classes, indexedWhelk, outputConfig.mode, specifiedProperties.isEmpty, outputConfig.outputClasses, outputConfig.outputIndividuals, queue, activeRestrictions, seenRefs))
       allTasks = ontologyDeclarationStream ++ classesTasks ++ restrictionsStream
     } yield allTasks.mapZIOParUnordered(JRuntime.getRuntime.availableProcessors)(identity)
     ZStream.unwrap(streamZ)
@@ -78,16 +78,16 @@ object RelationGraph extends StrictLogging {
 
   def allClasses(ont: OWLOntology): ZStream[Any, Nothing, OWLClass] = ZStream.fromIterable(ont.getClassesInSignature(Imports.INCLUDED).asScala.to(Set) - OWLThing - OWLNothing)
 
-  def traverse(specifiedProperties: Set[AtomicConcept], properties: Hierarchy, classes: Hierarchy, queue: Queue[Restriction], activeRestrictions: Ref[Int], seenRefs: Map[Role, Ref[Set[AtomicConcept]]]): UIO[Unit] = {
+  def traverse(specifiedProperties: Set[AtomicConcept], properties: Hierarchy, classes: Hierarchy, queue: Queue[Option[Restriction]], activeRestrictions: Ref[Int], seenRefs: Map[Role, Ref[Set[AtomicConcept]]]): UIO[Unit] = {
     val descendProperties = specifiedProperties.isEmpty
     val queryProperties = if (descendProperties) properties.subclasses.getOrElse(Top, Set.empty) - Bottom else specifiedProperties
     if (queryProperties.nonEmpty) ZIO.foreachParDiscard(queryProperties) { subprop =>
       traverseProperty(subprop, classes, queue, activeRestrictions, seenRefs)
     }
-    else queue.shutdown
+    else queue.offer(None).unit
   }
 
-  def traverseProperty(property: AtomicConcept, classes: Hierarchy, queue: Queue[Restriction], activeRestrictions: Ref[Int], seenRefs: Map[Role, Ref[Set[AtomicConcept]]]): UIO[Unit] = {
+  def traverseProperty(property: AtomicConcept, classes: Hierarchy, queue: Queue[Option[Restriction]], activeRestrictions: Ref[Int], seenRefs: Map[Role, Ref[Set[AtomicConcept]]]): UIO[Unit] = {
     val restrictions = (classes.subclasses.getOrElse(Top, Set.empty) - Bottom).map(filler => Restriction(Role(property.id), filler))
     val propSeenRef = seenRefs(Role(property.id))
     for {
@@ -95,13 +95,13 @@ object RelationGraph extends StrictLogging {
         seenForThisProperty ++ restrictions.map(_.filler)
       }
       _ <- activeRestrictions.update(current => current + restrictions.size)
-      _ <- queue.offerAll(restrictions).unit
+      _ <- queue.offerAll(restrictions.map(Option(_))).unit
       active <- activeRestrictions.get
-      _ <- queue.shutdown.when(active < 1)
+      _ <- queue.offer(None).when(active < 1).unit
     } yield ()
   }
 
-  def processRestrictionAndExtendQueue(restriction: Restriction, properties: Hierarchy, classes: Hierarchy, whelk: IndexedReasonerState, mode: Config.TriplesMode, descendProperties: Boolean, outputClasses: Boolean, outputIndividuals: Boolean, queue: Queue[Restriction], activeRestrictions: Ref[Int], seenRefs: Map[Role, Ref[Set[AtomicConcept]]]): UIO[TriplesGroup] = {
+  def processRestrictionAndExtendQueue(restriction: Restriction, properties: Hierarchy, classes: Hierarchy, whelk: IndexedReasonerState, mode: Config.TriplesMode, descendProperties: Boolean, outputClasses: Boolean, outputIndividuals: Boolean, queue: Queue[Option[Restriction]], activeRestrictions: Ref[Int], seenRefs: Map[Role, Ref[Set[AtomicConcept]]]): UIO[TriplesGroup] = {
     val triples = processRestriction(restriction, whelk, mode, outputClasses, outputIndividuals)
     val continue = triples.redundant.nonEmpty
     for {
@@ -128,9 +128,9 @@ object RelationGraph extends StrictLogging {
       } else ZIO.succeed(Set.empty[Restriction])
       newRestrictions = directFillerSubclassesRestrictions ++ directSubPropertyRestrictions
       _ <- activeRestrictions.update(current => current - 1 + newRestrictions.size)
-      _ <- queue.offerAll(newRestrictions)
+      _ <- queue.offerAll(newRestrictions.map(Option(_)))
       active <- activeRestrictions.get
-      _ <- queue.shutdown.when(active < 1)
+      _ <- queue.offer(None).when(active < 1).unit
     } yield triples
   }