diff --git a/lib/pdf/core_ext/mutex.rb b/lib/pdf/core_ext/mutex.rb
new file mode 100644
index 0000000..fa9e59d
--- /dev/null
+++ b/lib/pdf/core_ext/mutex.rb
@@ -0,0 +1,11 @@
+
+unless RUBY_VERSION < '1.9'
+  class ::Mutex
+    def marshal_dump
+    end unless method_defined?(:marshal_dump)
+    def marshal_load(data)
+    end unless method_defined?(:marshal_load)
+  end
+end
+
+
diff --git a/lib/pdf/writer.rb b/lib/pdf/writer.rb
index a861b3e..6f008b1 100644
--- a/lib/pdf/writer.rb
+++ b/lib/pdf/writer.rb
@@ -12,6 +12,7 @@
 require 'open-uri'
 
 require 'transaction/simple'
+require 'pdf/core_ext/mutex'
 require 'color'
 
   # A class to provide the core functionality to create a PDF document
diff --git a/pdf-writer.gemspec b/pdf-writer.gemspec
index ad3fb9a..56a2445 100644
--- a/pdf-writer.gemspec
+++ b/pdf-writer.gemspec
@@ -23,6 +23,8 @@ Gem::Specification.new do |s|
     "lib/pdf/charts",
     "lib/pdf/charts/stddev.rb",
     "lib/pdf/charts.rb",
+    "lib/pdf/core_ext",
+    "lib/pdf/core_ext/mutex.rb",
     "lib/pdf/math.rb",
     "lib/pdf/quickref.rb",
     "lib/pdf/simpletable.rb",