diff --git a/lib/rack/attack/configuration.rb b/lib/rack/attack/configuration.rb
index ba353ab4..c90fdee6 100644
--- a/lib/rack/attack/configuration.rb
+++ b/lib/rack/attack/configuration.rb
@@ -5,7 +5,7 @@
 module Rack
   class Attack
     class Configuration
-      DEFAULT_BLOCKLISTED_RESPONDER = lambda { |_req| [403, { 'Content-Type' => 'text/plain' }, ["Forbidden\n"]] }
+      DEFAULT_BLOCKLISTED_RESPONDER = lambda { |_req| [403, { 'content-type' => 'text/plain' }, ["Forbidden\n"]] }
 
       DEFAULT_THROTTLED_RESPONDER = lambda do |req|
         if Rack::Attack.configuration.throttled_response_retry_after_header
@@ -13,9 +13,9 @@ class Configuration
           now = match_data[:epoch_time]
           retry_after = match_data[:period] - (now % match_data[:period])
 
-          [429, { 'Content-Type' => 'text/plain', 'Retry-After' => retry_after.to_s }, ["Retry later\n"]]
+          [429, { 'content-type' => 'text/plain', 'retry-after' => retry_after.to_s }, ["Retry later\n"]]
         else
-          [429, { 'Content-Type' => 'text/plain' }, ["Retry later\n"]]
+          [429, { 'content-type' => 'text/plain' }, ["Retry later\n"]]
         end
       end