diff --git a/lib/mime.rb b/lib/mime.rb
index c738aa3..058997b 100644
--- a/lib/mime.rb
+++ b/lib/mime.rb
@@ -105,8 +105,11 @@ 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)
+			@body = [
+				preamble,
+				parts.map { |part| "\r\n" + part.to_s(opts).force_encoding('UTF-8') + "\r\n" },
+				"--\r\n" + epilogue
+			].flatten.join("\r\n--" + boundary)
   			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('; ')]