diff --git a/CHANGES.md b/CHANGES.md
index 7da895aad..f80e5cd72 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -24,6 +24,7 @@ Release Notes.
 * Add `spring-cloud-gateway-3.x` optional plugin.
 * Add `okhttp-4.x` plugin.
 * Fix NPE when thrift field is nested in plugin `thrift`
+* Fix possible NullPointerException in agent's ES plugin.
 
 #### OAP-Backend
 * BugFix: filter invalid Envoy access logs whose socket address is empty.
diff --git a/apm-sniffer/apm-sdk-plugin/elasticsearch-6.x-plugin/src/main/java/org/apache/skywalking/apm/plugin/elasticsearch/v6/define/RestHighLevelClientInstrumentation.java b/apm-sniffer/apm-sdk-plugin/elasticsearch-6.x-plugin/src/main/java/org/apache/skywalking/apm/plugin/elasticsearch/v6/define/RestHighLevelClientInstrumentation.java
index 7c2bbee4d..6adb02e39 100644
--- a/apm-sniffer/apm-sdk-plugin/elasticsearch-6.x-plugin/src/main/java/org/apache/skywalking/apm/plugin/elasticsearch/v6/define/RestHighLevelClientInstrumentation.java
+++ b/apm-sniffer/apm-sdk-plugin/elasticsearch-6.x-plugin/src/main/java/org/apache/skywalking/apm/plugin/elasticsearch/v6/define/RestHighLevelClientInstrumentation.java
@@ -257,4 +257,9 @@ public class RestHighLevelClientInstrumentation extends ClassEnhancePluginDefine
     public StaticMethodsInterceptPoint[] getStaticMethodsInterceptPoints() {
         return new StaticMethodsInterceptPoint[0];
     }
+
+    @Override
+    protected String[] witnessClasses() {
+        return new String[] {Constants.TASK_TRANSPORT_CHANNEL_WITNESS_CLASSES};
+    }
 }