diff --git a/lib/rack/livereload.rb b/lib/rack/livereload.rb
index 4fa99ac..89d1a52 100644
--- a/lib/rack/livereload.rb
+++ b/lib/rack/livereload.rb
@@ -54,7 +54,7 @@ module Rack
 
           body.each do |line|
             if !headers['X-Rack-LiveReload'] && line['</head>']
-              host_to_use = @options[:host] || env['HTTP_HOST'].gsub(%r{:.*}, '')
+              host_to_use = (@options[:host] || env['HTTP_HOST'] || 'localhost').gsub(%r{:.*}, '')
 
               if use_vendored?
                 src = LIVERELOAD_JS_PATH.dup + "?host=#{host_to_use}"
diff --git a/spec/rack/livereload_spec.rb b/spec/rack/livereload_spec.rb
index ed43d36..fb530b1 100644
--- a/spec/rack/livereload_spec.rb
+++ b/spec/rack/livereload_spec.rb
@@ -124,6 +124,14 @@ describe Rack::LiveReload do
         body.should_not include('web_socket')
       end
     end
+
+    context 'no host at all' do
+      let(:env) { {} }
+
+      it 'should use localhost' do
+        body.should include('localhost')
+      end
+    end
   end
 
   context '/__rack/livereload.js' do