diff --git a/Rakefile b/Rakefile
index f09cb7d..f2e659f 100644
--- a/Rakefile
+++ b/Rakefile
@@ -4,7 +4,8 @@ require 'rake/testtask'
 require 'rbconfig'
 require 'fileutils'
 
-spec = eval File.read('ruby-msg.gemspec')
+gemspec_file = File.expand_path(File.dirname(__FILE__) + '/ruby-msg.gemspec')
+spec = eval File.read(gemspec_file)
 
 task :default => [:test]
 
diff --git a/lib/mapi/mime.rb b/lib/mapi/mime.rb
index 7240489..c8b007a 100644
--- a/lib/mapi/mime.rb
+++ b/lib/mapi/mime.rb
@@ -106,8 +106,17 @@ def to_s opts={}
   		opts = {:boundary_counter => 0}.merge opts
   		if multipart?
   			boundary = Mime.make_boundary opts[:boundary_counter] += 1, self
-  			@body = [preamble, parts.map { |part| "\r\n" + part.to_s(opts) + "\r\n" }, "--\r\n" + epilogue].
-  				flatten.join("\r\n--" + boundary)
+        begin
+          @body = [preamble, parts.map { |part| "\r\n" + part.to_s(opts) + "\r\n" }, "--\r\n" + epilogue].
+            flatten.join("\r\n--" + boundary)
+        rescue Encoding::CompatibilityError => e
+          if ''.respond_to?(:force_encoding)
+            @body = [preamble, parts.map { |part| "\r\n" + part.to_s(opts).force_encoding('ASCII-8BIT') + "\r\n" }, "--\r\n" + epilogue].
+              flatten.join("\r\n--" + boundary)
+          else
+            raise e
+          end
+        end
   			content_type, attrs = Mime.split_header @headers['Content-Type'][0]
   			attrs['boundary'] = boundary
   			@headers['Content-Type'] = [([content_type] + attrs.map { |key, val| %{#{key}="#{val}"} }).join('; ')]
diff --git a/test/small-business-rates-relief.msg b/test/small-business-rates-relief.msg
new file mode 100644
index 0000000..1de1130
Binary files /dev/null and b/test/small-business-rates-relief.msg differ
diff --git a/test/test_msg.rb b/test/test_msg.rb
index ed52f0e..c1741b6 100644
--- a/test/test_msg.rb
+++ b/test/test_msg.rb
@@ -35,6 +35,15 @@ def test_rendered_string_is_valid_encoding
     end
   end
 
+  def test_multipart_rendered_string_is_valid_encoding
+    msg = Mapi::Msg.open "#{TEST_DIR}/small-business-rates-relief.msg" do |msg|
+      string_version = msg.to_mime.to_s
+      if string_version.respond_to?(:valid_encoding?)
+        assert_equal true, string_version.valid_encoding?
+      end
+    end
+  end
+
   def test_embedded_msg_renders_as_string
     msg = Mapi::Msg.open "#{TEST_DIR}/embedded.msg" do |msg|
       assert_match "message/rfc822", msg.to_mime.to_s