diff --git a/lib/sinatra/base.rb b/lib/sinatra/base.rb
index c302031df2..168c34149f 100644
--- a/lib/sinatra/base.rb
+++ b/lib/sinatra/base.rb
@@ -358,7 +358,7 @@ def content_type(type = nil, params = {})
     # Set the Content-Disposition to "attachment" with the specified filename,
     # instructing the user agents to prompt to save.
     def attachment(filename = nil, disposition = :attachment)
-      response['Content-Disposition'] = disposition.to_s
+      response['Content-Disposition'] = disposition.to_s.dup
       if filename
         params = '; filename="%s"' % File.basename(filename)
         response['Content-Disposition'] << params
diff --git a/test/helpers_test.rb b/test/helpers_test.rb
index 3575e945cd..86d6d0a438 100644
--- a/test/helpers_test.rb
+++ b/test/helpers_test.rb
@@ -879,6 +879,12 @@ def send_file_app(opts={})
       assert_equal 'inline; filename="file.txt"', response['Content-Disposition']
     end
 
+    it "does not raise an error when :disposition set to a frozen string" do
+      send_file_app :disposition => 'inline'.freeze
+      get '/file.txt'
+      assert_equal 'inline; filename="file.txt"', response['Content-Disposition']
+    end 
+    
     it "sets the Content-Disposition header when :filename provided" do
       send_file_app :filename => 'foo.txt'
       get '/file.txt'