Potential NPE in pivot facet

For some queries distributed over multiple collections, I've hit a NPE when SolrIndexSearcher tries to fetch results from cache. Basically, query generated to compute pivot on document sub set is null, causing the NPE on lookup.

2016-03-28 11:34:58.361 ERROR (qtp268141378-751) [c:otif_fr s:shard1 r:core_node1 x:otif_fr_shard1_replica1] o.a.s.h.RequestHandlerBase java.lang.NullPointerException
at java.util.concurrent.ConcurrentHashMap.get(ConcurrentHashMap.java:936)
at org.apache.solr.util.ConcurrentLFUCache.get(ConcurrentLFUCache.java:92)
at org.apache.solr.search.LFUCache.get(LFUCache.java:153)
at org.apache.solr.search.SolrIndexSearcher.getPositiveDocSet(SolrIndexSearcher.java:940)
at org.apache.solr.search.SolrIndexSearcher.numDocs(SolrIndexSearcher.java:2098)
at org.apache.solr.handler.component.PivotFacetProcessor.getSubsetSize(PivotFacetProcessor.java:356)
at org.apache.solr.handler.component.PivotFacetProcessor.processSingle(PivotFacetProcessor.java:219)
at org.apache.solr.handler.component.PivotFacetProcessor.process(PivotFacetProcessor.java:167)
at org.apache.solr.handler.component.FacetComponent.process(FacetComponent.java:263)
at org.apache.solr.handler.component.SearchHandler.handleRequestBody(SearchHandler.java:273)
at org.apache.solr.handler.RequestHandlerBase.handleRequest(RequestHandlerBase.java:156)
at org.apache.solr.core.SolrCore.execute(SolrCore.java:2073)
at org.apache.solr.servlet.HttpSolrCall.execute(HttpSolrCall.java:658)
at org.apache.solr.servlet.HttpSolrCall.call(HttpSolrCall.java:457)
at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:223)
at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:181)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:585)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:577)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:223)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1127)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:515)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:185)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1061)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:215)
at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:110)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97)
at org.eclipse.jetty.server.Server.handle(Server.java:499)
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:310)
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:257)
at org.eclipse.jetty.io.AbstractConnection$2.run(AbstractConnection.java:540)
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:635)
at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:555)
at java.lang.Thread.run(Thread.java:745)