diff --git a/lib/puma/const.rb b/lib/puma/const.rb
index cfa9d922..c56ca45f 100644
--- a/lib/puma/const.rb
+++ b/lib/puma/const.rb
@@ -86,6 +86,7 @@ module Puma
     SERVER_PORT = "SERVER_PORT".freeze
     HTTP_HOST = "HTTP_HOST".freeze
     PORT_80 = "80".freeze
+    LOCALHOST = "localhost".freeze
 
     SERVER_PROTOCOL = "SERVER_PROTOCOL".freeze
     HTTP_11 = "HTTP/1.1".freeze
diff --git a/lib/puma/server.rb b/lib/puma/server.rb
index c7912bd4..5d8cab9f 100644
--- a/lib/puma/server.rb
+++ b/lib/puma/server.rb
@@ -356,6 +356,9 @@ module Puma
           env[SERVER_NAME] = host
           env[SERVER_PORT] = PORT_80
         end
+      else
+        env[SERVER_NAME] = LOCALHOST
+        env[SERVER_PORT] = PORT_80
       end
 
       unless env[REQUEST_PATH]