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 9137e63..520ae0a 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/ExportReviewNotes.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/ExportReviewNotes.java
@@ -17,6 +17,7 @@
 import com.google.common.collect.ImmutableListMultimap;
 import com.google.common.collect.ListMultimap;
 import com.google.common.collect.Maps;
+import com.google.common.collect.MultimapBuilder;
 import com.google.gerrit.reviewdb.client.Change;
 import com.google.gerrit.reviewdb.client.Project;
 import com.google.gerrit.reviewdb.server.ReviewDb;
@@ -72,8 +73,11 @@
 
   private ListMultimap<Project.NameKey, ChangeNotes> mergedChanges() {
     try (ReviewDb db = database.open()) {
-      return notesFactory.create(
-          db, notes -> notes.getChange().getStatus() == Change.Status.MERGED);
+      return MultimapBuilder.hashKeys()
+          .arrayListValues()
+          .build(
+              notesFactory.create(
+                  db, notes -> notes.getChange().getStatus() == Change.Status.MERGED));
     } catch (OrmException | IOException e) {
       stderr.println("Cannot read changes from database " + e.getMessage());
       return ImmutableListMultimap.of();