diff --git a/solr/solrj/src/java/org/apache/solr/client/solrj/impl/ConcurrentUpdateHttp2SolrClient.java b/solr/solrj/src/java/org/apache/solr/client/solrj/impl/ConcurrentUpdateHttp2SolrClient.java
index ea0f773edc8..0883e7fe048 100644
--- a/solr/solrj/src/java/org/apache/solr/client/solrj/impl/ConcurrentUpdateHttp2SolrClient.java
+++ b/solr/solrj/src/java/org/apache/solr/client/solrj/impl/ConcurrentUpdateHttp2SolrClient.java
@@ -329,7 +329,7 @@ public class ConcurrentUpdateHttp2SolrClient extends SolrClient {
 
   // *must* be called with runners monitor held, e.g. synchronized(runners){ addRunner() }
   private void addRunner() {
-    MDC.put("ConcurrentUpdateHttp2SolrClient.url", client.getBaseURL());
+    MDC.put("ConcurrentUpdateHttp2SolrClient.url", String.valueOf(client.getBaseURL())); // MDC can't have null value
     try {
       Runner r = new Runner();
       runners.add(r);