diff --git a/app/services/user_merger.rb b/app/services/user_merger.rb
index 600189565de..b9fe2bb7732 100644
--- a/app/services/user_merger.rb
+++ b/app/services/user_merger.rb
@@ -355,7 +355,7 @@ class UserMerger
                    conditions: ["x.action_type = y.action_type",
                                 "x.target_topic_id IS NOT DISTINCT FROM y.target_topic_id",
                                 "x.target_post_id IS NOT DISTINCT FROM y.target_post_id",
-                                "x.acting_user_id IN (:source_user_id, :target_user_id)"])
+                                "(x.acting_user_id IN (:source_user_id, :target_user_id) OR x.acting_user_id IS NOT DISTINCT FROM y.acting_user_id)"])
     update_user_id(:user_actions,
                    user_id_column_name: "acting_user_id",
                    conditions: ["x.action_type = y.action_type",
diff --git a/spec/services/user_merger_spec.rb b/spec/services/user_merger_spec.rb
index f74abc38625..ac5d3545b43 100644
--- a/spec/services/user_merger_spec.rb
+++ b/spec/services/user_merger_spec.rb
@@ -702,7 +702,7 @@ describe UserMerger do
 
   context "user actions" do
     # action_type and user_id are not nullable
-    # target_topic_id and acting_user_id are nullable, but always a value
+    # target_topic_id and acting_user_id are nullable, but always have a value
 
     let(:post1) { Fabricate(:post) }
     let(:post2) { Fabricate(:post) }
@@ -724,6 +724,14 @@ describe UserMerger do
                              target_post_id: post.id)
     end
 
+    def log_got_private_message(acting_user, user, topic)
+      UserAction.log_action!(action_type: UserAction::GOT_PRIVATE_MESSAGE,
+                             user_id: user.id,
+                             acting_user_id: acting_user.id,
+                             target_topic_id: topic.id,
+                             target_post_id: -1)
+    end
+
     it "merges when target_post_id is not set" do
       a1 = log_pending_action(source_user, post1)
       a2 = log_pending_action(source_user, post2)
@@ -754,6 +762,42 @@ describe UserMerger do
                                     acting_user_id: target_user.id).pluck(:id)
       expect(action_ids).to contain_exactly(a2.id, a3.id)
     end
+
+    it "merges when acting_user is neither source_user nor target_user" do
+      coding_horror = Fabricate(:coding_horror)
+
+      pm_topic1 = Fabricate(:private_message_topic, topic_allowed_users: [
+        Fabricate.build(:topic_allowed_user, user: walter),
+        Fabricate.build(:topic_allowed_user, user: source_user),
+        Fabricate.build(:topic_allowed_user, user: target_user),
+        Fabricate.build(:topic_allowed_user, user: coding_horror),
+      ])
+
+      pm_topic2 = Fabricate(:private_message_topic, topic_allowed_users: [
+        Fabricate.build(:topic_allowed_user, user: walter),
+        Fabricate.build(:topic_allowed_user, user: source_user)
+      ])
+
+      pm_topic3 = Fabricate(:private_message_topic, topic_allowed_users: [
+        Fabricate.build(:topic_allowed_user, user: walter),
+        Fabricate.build(:topic_allowed_user, user: target_user)
+      ])
+
+      a1 = log_got_private_message(walter, source_user, pm_topic1)
+      a2 = log_got_private_message(walter, target_user, pm_topic1)
+      a3 = log_got_private_message(walter, coding_horror, pm_topic1)
+      a4 = log_got_private_message(walter, source_user, pm_topic2)
+      a5 = log_got_private_message(walter, target_user, pm_topic3)
+
+      merge_users!
+
+      expect(UserAction.count).to eq(4)
+
+      action_ids = UserAction.where(action_type: UserAction::GOT_PRIVATE_MESSAGE,
+                                    user_id: target_user.id,
+                                    acting_user_id: walter.id).pluck(:id)
+      expect(action_ids).to contain_exactly(a2.id, a4.id, a5.id)
+    end
   end
 
   it "merges archived messages" do