Null pointer exception when

This can be reproduce by certain inputs, but I can't find a pattern in the inputs that cause this error:

Caused by: org.apache.solr.client.solrj.impl.HttpSolrClient$RemoteSolrException: Error from server at http://192.168.1.47:8983/solr/core1: java.lang.NullPointerException
at org.apache.solr.search.facet.FacetFieldProcessorFCBase.findTopSlots(FacetField.java:301)
at org.apache.solr.search.facet.FacetFieldProcessorFCBase.getFieldCacheCounts(FacetField.java:254)
at org.apache.solr.search.facet.FacetFieldProcessorFCBase.process(FacetField.java:218)
at org.apache.solr.search.facet.FacetProcessor.processSubs(FacetRequest.java:267)
at org.apache.solr.search.facet.FacetFieldProcessorNumeric.calcFacets(FacetFieldProcessorNumeric.java:472)
at org.apache.solr.search.facet.FacetFieldProcessorNumeric.process(FacetFieldProcessorNumeric.java:151)
at org.apache.solr.search.facet.FacetProcessor.processSubs(FacetRequest.java:267)
at org.apache.solr.search.facet.FacetProcessor.fillBucket(FacetRequest.java:354)
at org.apache.solr.search.facet.FacetQueryProcessor.process(FacetQuery.java:57)
at org.apache.solr.search.facet.FacetModule.process(FacetModule.java:87)
at org.apache.solr.handler.component.SearchHandler.handleRequestBody(SearchHandler.java:255)
at org.apache.solr.handler.RequestHandlerBase.handleRequest(RequestHandlerBase.java:143)
at org.apache.solr.core.SolrCore.execute(SolrCore.java:2064)
at org.apache.solr.servlet.HttpSolrCall.execute(HttpSolrCall.java:654)
at org.apache.solr.servlet.HttpSolrCall.call(HttpSolrCall.java:450)
at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:227)
at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:196)
at org.eclipse.jetty.servlet.ServletHandler$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:497)
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)