diff --git a/lib/rack/attack.rb b/lib/rack/attack.rb
index e51550b..77a9b53 100644
--- a/lib/rack/attack.rb
+++ b/lib/rack/attack.rb
@@ -35,7 +35,7 @@ module Rack::Attack
       @blacklisted_response ||= lambda {|env| [503, {}, ['Blocked']] }
       @throttled_response   ||= lambda {|env|
         retry_after = env['rack.attack.match_data'][:period] rescue nil
-        [503, {'Retry-After' => retry_after}, ['Retry later']]
+        [503, {'Retry-After' => retry_after.to_s}, ['Retry later']]
       }
 
       self
diff --git a/spec/rack_attack_spec.rb b/spec/rack_attack_spec.rb
index 9a68bfc..8a56f98 100644
--- a/spec/rack_attack_spec.rb
+++ b/spec/rack_attack_spec.rb
@@ -93,7 +93,7 @@ describe 'Rack::Attack' do
         last_request.env['rack.attack.match_data'].must_equal({:count => 2, :limit => 1, :period => 1})
       end
       it 'should set a Retry-After header' do
-        last_response.headers['Retry-After'].must_equal 1
+        last_response.headers['Retry-After'].must_equal '1'
       end
     end