diff --git a/hbase-server/src/main/java/org/apache/hadoop/hbase/master/HMaster.java b/hbase-server/src/main/java/org/apache/hadoop/hbase/master/HMaster.java
index 3f13a45c360..18de2fdf796 100644
--- a/hbase-server/src/main/java/org/apache/hadoop/hbase/master/HMaster.java
+++ b/hbase-server/src/main/java/org/apache/hadoop/hbase/master/HMaster.java
@@ -845,11 +845,6 @@ public class HMaster extends HRegionServer implements MasterServices {
    */
   private void finishActiveMasterInitialization(MonitoredTask status)
       throws IOException, InterruptedException, KeeperException, ReplicationException {
-    Thread zombieDetector = new Thread(new InitializationMonitor(this),
-        "ActiveMasterInitializationMonitor-" + System.currentTimeMillis());
-    zombieDetector.setDaemon(true);
-    zombieDetector.start();
-
     /*
      * We are active master now... go initialize components we need to run.
      */
@@ -919,6 +914,12 @@ public class HMaster extends HRegionServer implements MasterServices {
     // Set ourselves as active Master now our claim has succeeded up in zk.
     this.activeMaster = true;
 
+    // Start the Zombie master detector after setting master as active, see HBASE-21535
+    Thread zombieDetector = new Thread(new InitializationMonitor(this),
+        "ActiveMasterInitializationMonitor-" + System.currentTimeMillis());
+    zombieDetector.setDaemon(true);
+    zombieDetector.start();
+
     // This is for backwards compatibility
     // See HBASE-11393
     status.setStatus("Update TableCFs node in ZNode");