app/controllers/api/v1/tags_controller.rb

@@ -12,7 +12,7 @@ class Api::V1::TagsController < Api::BaseController
   end
 
   def follow
-    TagFollow.first_or_create!(tag: @tag, account: current_account, rate_limit: true)
+    TagFollow.create_with(rate_limit: true).find_or_create_by!(tag: @tag, account: current_account)
     render json: @tag, serializer: REST::TagSerializer
   end
 

spec/controllers/api/v1/tags_controller_spec.rb

@@ -33,7 +33,11 @@ RSpec.describe Api::V1::TagsController, type: :controller do
   end
 
   describe 'POST #follow' do
+    let!(:unrelated_tag) { Fabricate(:tag) }
+
     before do
+      TagFollow.create!(account: user.account, tag: unrelated_tag)
+
       post :follow, params: { id: name }
     end