diff --git a/app/models/watched_word.rb b/app/models/watched_word.rb
index f08f523b1c8..fafbdd388a6 100644
--- a/app/models/watched_word.rb
+++ b/app/models/watched_word.rb
@@ -28,6 +28,7 @@ class WatchedWord < ActiveRecord::Base
   validates :action, presence: true
 
   validate :replacement_is_url, if: -> { action == WatchedWord.actions[:link] }
+  validate :replacement_is_tag_list, if: -> { action == WatchedWord.actions[:tag] }
 
   validates_each :word do |record, attr, val|
     if WatchedWord.where(action: record.action).count >= MAX_WORDS_PER_ACTION
@@ -50,6 +51,14 @@ class WatchedWord < ActiveRecord::Base
     end
   end
 
+  def replacement_is_tag_list
+    tag_list = replacement&.split(',')
+    tags = Tag.where(name: tag_list)
+    if (tag_list.blank? || tags.empty? || tag_list.size != tags.size)
+      errors.add(:base, :invalid_tag_list)
+    end
+  end
+
   def self.create_or_update_word(params)
     new_word = normalize_word(params[:word])
     w = WatchedWord.where("word ILIKE ?", new_word).first || WatchedWord.new(word: new_word)
diff --git a/config/locales/server.en.yml b/config/locales/server.en.yml
index 31fd766d6e1..d1356341edd 100644
--- a/config/locales/server.en.yml
+++ b/config/locales/server.en.yml
@@ -639,6 +639,7 @@ en:
               too_many: "Too many words for that action"
             base:
               invalid_url: "Replacement URL is invalid"
+              invalid_tag_list: "Replacement tag list is invalid"
 
   uncategorized_category_name: "Uncategorized"
 
diff --git a/spec/lib/post_creator_spec.rb b/spec/lib/post_creator_spec.rb
index 1c929c26095..237c91b3eff 100644
--- a/spec/lib/post_creator_spec.rb
+++ b/spec/lib/post_creator_spec.rb
@@ -529,11 +529,15 @@ describe PostCreator do
             before do
               SiteSetting.min_trust_to_create_tag = 0
               SiteSetting.min_trust_level_to_tag_topics = 0
+              Fabricate(:tag, name: 'greetings')
+              Fabricate(:tag, name: 'hey')
+              Fabricate(:tag, name: 'about-art')
+              Fabricate(:tag, name: 'about-artists')
             end
 
             context "without regular expressions" do
               it "works with many tags" do
-                Fabricate(:watched_word, action: WatchedWord.actions[:tag], word: "HELLO", replacement: "greetings , hey")
+                Fabricate(:watched_word, action: WatchedWord.actions[:tag], word: "HELLO", replacement: "greetings,hey")
 
                 @post = creator.create
                 expect(@post.topic.tags.map(&:name)).to match_array(['greetings', 'hey'])
@@ -548,7 +552,7 @@ describe PostCreator do
               end
 
               it "does not treat as regular expressions" do
-                Fabricate(:watched_word, action: WatchedWord.actions[:tag], word: "he(llo|y)", replacement: "greetings , hey")
+                Fabricate(:watched_word, action: WatchedWord.actions[:tag], word: "he(llo|y)", replacement: "greetings,hey")
 
                 @post = creator_with_tags.create
                 expect(@post.topic.tags.map(&:name)).to match_array(tag_names)
@@ -558,7 +562,7 @@ describe PostCreator do
             context "with regular expressions" do
               it "works" do
                 SiteSetting.watched_words_regular_expressions = true
-                Fabricate(:watched_word, action: WatchedWord.actions[:tag], word: "he(llo|y)", replacement: "greetings , hey")
+                Fabricate(:watched_word, action: WatchedWord.actions[:tag], word: "he(llo|y)", replacement: "greetings,hey")
 
                 @post = creator_with_tags.create
                 expect(@post.topic.tags.map(&:name)).to match_array(tag_names + ['greetings', 'hey'])
diff --git a/spec/models/watched_word_spec.rb b/spec/models/watched_word_spec.rb
index 60ed4e1c66b..f43ec6a774f 100644
--- a/spec/models/watched_word_spec.rb
+++ b/spec/models/watched_word_spec.rb
@@ -89,6 +89,12 @@ describe WatchedWord do
       }.to_not change { described_class.count }
     end
 
+    it "error when an tag action is created without valid tags" do
+      expect {
+        described_class.create!(word: "ramones", action: described_class.actions[:tag])
+      }.to raise_error(ActiveRecord::RecordInvalid)
+    end
+
     it "replaces link with absolute URL" do
       word = Fabricate(:watched_word, action: described_class.actions[:link], word: "meta1")
       expect(word.replacement).to eq("http://test.localhost/")
diff --git a/spec/requests/admin/watched_words_controller_spec.rb b/spec/requests/admin/watched_words_controller_spec.rb
index 5d1c6b3e74a..09930880234 100644
--- a/spec/requests/admin/watched_words_controller_spec.rb
+++ b/spec/requests/admin/watched_words_controller_spec.rb
@@ -30,6 +30,9 @@ RSpec.describe Admin::WatchedWordsController do
     context 'logged in as admin' do
       before do
         sign_in(admin)
+        Fabricate(:tag, name: 'tag1')
+        Fabricate(:tag, name: 'tag2')
+        Fabricate(:tag, name: 'tag3')
       end
 
       it 'creates the words from the file' do
@@ -80,6 +83,9 @@ RSpec.describe Admin::WatchedWordsController do
     context 'logged in as admin' do
       before do
         sign_in(admin)
+        Fabricate(:tag, name: 'tag1')
+        Fabricate(:tag, name: 'tag2')
+        Fabricate(:tag, name: 'tag3')
       end
 
       it "words of different actions are downloaded separately" do