diff --git a/app/models/channel/filter/follow_up_check.rb b/app/models/channel/filter/follow_up_check.rb
index 50346428e..7024b195b 100644
--- a/app/models/channel/filter/follow_up_check.rb
+++ b/app/models/channel/filter/follow_up_check.rb
@@ -14,7 +14,7 @@ module Channel::Filter::FollowUpCheck
       return true
     end
 
-    setting = Setting.get('postmaster_follow_up_search_in')
+    setting = Setting.get('postmaster_follow_up_search_in') || []
 
     # get ticket# from body
     if setting.include?('body')
diff --git a/test/unit/email_process_follow_up_test.rb b/test/unit/email_process_follow_up_test.rb
index f7c12a2bb..b324a5545 100644
--- a/test/unit/email_process_follow_up_test.rb
+++ b/test/unit/email_process_follow_up_test.rb
@@ -104,6 +104,50 @@ no reference "
     ticket_p, article_p, user_p = Channel::EmailParser.new.process({}, email_raw_string_references2)
     assert_equal(ticket.id, ticket_p.id)
 
+    Setting.set('postmaster_follow_up_search_in', nil)
+
+    travel 1.second
+    ticket_p, article_p, user_p = Channel::EmailParser.new.process({}, email_raw_string_subject)
+    assert_equal(ticket.id, ticket_p.id)
+
+    travel 1.second
+    ticket_p, article_p, user_p = Channel::EmailParser.new.process({}, email_raw_string_body)
+    assert_not_equal(ticket.id, ticket_p.id)
+
+    travel 1.second
+    ticket_p, article_p, user_p = Channel::EmailParser.new.process({}, email_raw_string_attachment)
+    assert_not_equal(ticket.id, ticket_p.id)
+
+    travel 1.second
+    ticket_p, article_p, user_p = Channel::EmailParser.new.process({}, email_raw_string_references1)
+    assert_not_equal(ticket.id, ticket_p.id)
+
+    travel 1.second
+    ticket_p, article_p, user_p = Channel::EmailParser.new.process({}, email_raw_string_references2)
+    assert_not_equal(ticket.id, ticket_p.id)
+
+    Setting.set('postmaster_follow_up_search_in', 'references')
+
+    travel 1.second
+    ticket_p, article_p, user_p = Channel::EmailParser.new.process({}, email_raw_string_subject)
+    assert_equal(ticket.id, ticket_p.id)
+
+    travel 1.second
+    ticket_p, article_p, user_p = Channel::EmailParser.new.process({}, email_raw_string_body)
+    assert_not_equal(ticket.id, ticket_p.id)
+
+    travel 1.second
+    ticket_p, article_p, user_p = Channel::EmailParser.new.process({}, email_raw_string_attachment)
+    assert_not_equal(ticket.id, ticket_p.id)
+
+    travel 1.second
+    ticket_p, article_p, user_p = Channel::EmailParser.new.process({}, email_raw_string_references1)
+    assert_equal(ticket.id, ticket_p.id)
+
+    travel 1.second
+    ticket_p, article_p, user_p = Channel::EmailParser.new.process({}, email_raw_string_references2)
+    assert_equal(ticket.id, ticket_p.id)
+
     Setting.set('postmaster_follow_up_search_in', setting_orig)
 
     travel 1.second