Null pointerException in AbstractStatsValues.accumulate

It happens when there is a string field with docValues="true" and default="".
Then, with documents that have empty string value in the field,
values.exists(docID) is true but values.strVal(docID) is null, and it throws null pointer exception when trying to add the value to distinctValues set.
the solr query is stats=true&stats.field=X&stats.calcdistinct=true

stack trace:
java.lang.NullPointerException at java.util.TreeMap.put(TreeMap.java:567) at java.util.TreeSet.add(TreeSet.java:266) at org.apache.solr.handler.component.AbstractStatsValues.accumulate(StatsValuesFactory.java:164) at org.apache.solr.handler.component.StringStatsValues.accumulate(StatsValuesFactory.java:535) at org.apache.solr.handler.component.SimpleStats.getFieldCacheStats(StatsComponent.java:274) at org.apache.solr.handler.component.SimpleStats.getStatsFields(StatsComponent.java:225) at org.apache.solr.handler.component.SimpleStats.getStatsCounts(StatsComponent.java:200) at org.apache.solr.handler.component.StatsComponent.process(StatsComponent.java:68) at org.apache.solr.handler.component.SearchHandler.handleRequestBody(SearchHandler.java:208) at org.apache.solr.handler.RequestHandlerBase.handleRequest(RequestHandlerBase.java:135) at org.apache.solr.core.SolrCore.execute(SolrCore.java:1904) at org.apache.solr.servlet.SolrDispatchFilter.execute(SolrDispatchFilter.java:659) at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:362) at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:158) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1474) at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:499) at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:137) at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:557) at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:231) at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1086) at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:428) at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:193) at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1020) at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:135) at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:255) at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:154) at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:116) at org.eclipse.jetty.server.Server.handle(Server.java:370) at org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:489) at org.eclipse.jetty.server.AbstractHttpConnection.headerComplete(AbstractHttpConnection.java:949) at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.headerComplete(AbstractHttpConnection.java:1011) at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:644) at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:235) at org.eclipse.jetty.server.BlockingHttpConnection.handle(BlockingHttpConnection.java:72) at org.eclipse.jetty.server.bio.SocketConnector$ConnectorEndPoint.run(SocketConnector.java:264) at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:608) at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:543) at java.lang.Thread.run(Thread.java:804)