diff --git a/driver-core/src/main/com/mongodb/connection/DefaultServerMonitor.java b/driver-core/src/main/com/mongodb/connection/DefaultServerMonitor.java
index 3f0ffb6e547..c68c7504939 100644
--- a/driver-core/src/main/com/mongodb/connection/DefaultServerMonitor.java
+++ b/driver-core/src/main/com/mongodb/connection/DefaultServerMonitor.java
@@ -89,7 +89,7 @@ public void connect() {
     }
 
     @Override
-    public void invalidate() {
+    public synchronized void invalidate() {
         isTrue("open", !isClosed);
         monitor.close();
         monitorThread.interrupt();