diff --git a/lib/sinatra/base.rb b/lib/sinatra/base.rb
index 406008f9..211dfefc 100644
--- a/lib/sinatra/base.rb
+++ b/lib/sinatra/base.rb
@@ -240,6 +240,7 @@ module Sinatra
     # matches the time specified, execution is immediately halted with a
     # '304 Not Modified' response.
     def last_modified(time)
+      return unless time
       time = time.to_time if time.respond_to?(:to_time)
       time = time.httpdate if time.respond_to?(:httpdate)
       response['Last-Modified'] = time
diff --git a/test/helpers_test.rb b/test/helpers_test.rb
index 7c8c0920..2671ec95 100644
--- a/test/helpers_test.rb
+++ b/test/helpers_test.rb
@@ -451,6 +451,15 @@ class HelpersTest < Test::Unit::TestCase
       assert_equal 304, status
       assert_equal '', body
     end
+
+    it 'ignores nil' do
+      mock_app {
+        get '/' do last_modified nil; 200; end
+      }
+
+      get '/'
+      assert ! response['Last-Modified']
+    end
   end
 
   describe 'etag' do