diff --git a/.gitignore b/.gitignore
index a955cfa..52bf0af 100644
--- a/.gitignore
+++ b/.gitignore
@@ -16,3 +16,4 @@ test/tmp
 test/version_tmp
 tmp
 .DS_Store
+.idea
\ No newline at end of file
diff --git a/lib/rack/tracker/google_tag_manager/google_tag_manager.rb b/lib/rack/tracker/google_tag_manager/google_tag_manager.rb
index 2db9d2b..46a5821 100644
--- a/lib/rack/tracker/google_tag_manager/google_tag_manager.rb
+++ b/lib/rack/tracker/google_tag_manager/google_tag_manager.rb
@@ -10,13 +10,12 @@ def inject(response)
     # Sub! is enough, in well formed html there's only one head or body tag.
     # Block syntax need to be used, otherwise backslashes in input will mess the output.
     # @see http://stackoverflow.com/a/4149087/518204 and https://github.com/railslove/rack-tracker/issues/50
-    response.sub! %r{<head.*>} do |m|
+    response_dup = response.sub %r{<head.*>} do |m|
       m.to_s << self.render_head
     end
-    response.sub! %r{<body.*?>} do |m|
+    response_dup.sub %r{<body.*?>} do |m|
       m.to_s << self.render_body
     end
-    response
   end
 
   def container
diff --git a/lib/rack/tracker/handler.rb b/lib/rack/tracker/handler.rb
index 25c7788..c203791 100644
--- a/lib/rack/tracker/handler.rb
+++ b/lib/rack/tracker/handler.rb
@@ -46,10 +46,9 @@ def inject(response)
     # Sub! is enough, in well formed html there's only one head or body tag.
     # Block syntax need to be used, otherwise backslashes in input will mess the output.
     # @see http://stackoverflow.com/a/4149087/518204 and https://github.com/railslove/rack-tracker/issues/50
-    response.sub! %r{</#{self.position}>} do |m|
+    response.sub %r{</#{self.position}>} do |m|
       self.render << m.to_s
     end
-    response
   end
 
   def write_event(event)