diff --git a/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/ExportReviewNotes.java b/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/ExportReviewNotes.java
index 06975a0..447cb77 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/ExportReviewNotes.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/ExportReviewNotes.java
@@ -50,6 +50,8 @@
 
   @Inject private RetryHelper retryHelper;
 
+  private static final Object lock = new Object();
+
   private ListMultimap<Project.NameKey, ChangeNotes> changes;
   private ThreadSafeProgressMonitor monitor;
 
@@ -107,7 +109,7 @@
   }
 
   private Map.Entry<Project.NameKey, List<ChangeNotes>> next() {
-    synchronized (changes) {
+    synchronized (lock) {
       if (changes.isEmpty()) {
         return null;
       }