NullPointerException in JSON facet

java.lang.NullPointerException
at org.apache.solr.search.facet.AvgSlotAcc.collect(SlotAcc.java:390)
at org.apache.solr.search.facet.FacetFieldProcessor$MultiAcc.collect(FacetFieldProcessor.java:510)
at org.apache.solr.search.facet.SlotAcc.collect(SlotAcc.java:97)
at org.apache.solr.search.facet.FacetFieldProcessor.collectFirstPhase(FacetFieldProcessor.java:220)
at org.apache.solr.search.facet.UnInvertedField.collectDocsGeneric(UnInvertedField.java:431)
at org.apache.solr.search.facet.UnInvertedField.collectDocs(UnInvertedField.java:410)
at org.apache.solr.search.facet.FacetFieldProcessorByArrayUIF.collectDocs(FacetFieldProcessorByArrayUIF.java:64)
at org.apache.solr.search.facet.FacetFieldProcessorByArray.calcFacets(FacetFieldProcessorByArray.java:108)
at org.apache.solr.search.facet.FacetFieldProcessorByArray.process(FacetFieldProcessorByArray.java:58)
at org.apache.solr.search.facet.FacetProcessor.processSubs(FacetProcessor.java:460)
at org.apache.solr.search.facet.FacetProcessor.fillBucket(FacetProcessor.java:407)
at org.apache.solr.search.facet.FacetQueryProcessor.process(FacetQuery.java:64)
at org.apache.solr.search.facet.FacetModule.process(FacetModule.java:154)
at org.apache.solr.handler.component.SearchHandler.handleRequestBody(SearchHandler.java:295)
at org.apache.solr.handler.RequestHandlerBase.handleRequest(RequestHandlerBase.java:180)
at org.apache.solr.core.SolrCore.execute(SolrCore.java:2503)
at org.apache.solr.servlet.HttpSolrCall.execute(HttpSolrCall.java:710)
at org.apache.solr.servlet.HttpSolrCall.call(HttpSolrCall.java:516)
at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:382)
at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:326)