diff --git a/lib/execjs/external_runtime.rb b/lib/execjs/external_runtime.rb
index ec06d3f..5ed9272 100644
--- a/lib/execjs/external_runtime.rb
+++ b/lib/execjs/external_runtime.rb
@@ -6,14 +6,14 @@ module ExecJS
   class ExternalRuntime
     class Context
       def initialize(runtime, source = "")
-        source = source.encode('UTF-8') if source.respond_to?(:encode)
+        source = ExecJS::encode(source) if source.respond_to?(:encode)
 
         @runtime = runtime
         @source  = source
       end
 
       def eval(source, options = {})
-        source = source.encode('UTF-8') if source.respond_to?(:encode)
+        source = ExecJS::encode(source) if source.respond_to?(:encode)
 
         if /\S/ =~ source
           exec("return eval(#{json_encode("(#{source})")})")
@@ -21,7 +21,7 @@ module ExecJS
       end
 
       def exec(source, options = {})
-        source = source.encode('UTF-8') if source.respond_to?(:encode)
+        source = ExecJS::encode(source) if source.respond_to?(:encode)
         source = "#{@source}\n#{source}" if @source
 
         compile_to_tempfile(source) do |file|
diff --git a/lib/execjs/ruby_rhino_runtime.rb b/lib/execjs/ruby_rhino_runtime.rb
index df55395..2113941 100644
--- a/lib/execjs/ruby_rhino_runtime.rb
+++ b/lib/execjs/ruby_rhino_runtime.rb
@@ -2,7 +2,7 @@ module ExecJS
   class RubyRhinoRuntime
     class Context
       def initialize(source = "")
-        source = source.encode('UTF-8') if source.respond_to?(:encode)
+        source = ExecJS::encode(source) if source.respond_to?(:encode)
 
         @rhino_context = ::Rhino::Context.new
         fix_memory_limit! @rhino_context
@@ -10,7 +10,7 @@ module ExecJS
       end
 
       def exec(source, options = {})
-        source = source.encode('UTF-8') if source.respond_to?(:encode)
+        source = ExecJS::encode(source) if source.respond_to?(:encode)
 
         if /\S/ =~ source
           eval "(function(){#{source}})()", options
@@ -18,7 +18,7 @@ module ExecJS
       end
 
       def eval(source, options = {})
-        source = source.encode('UTF-8') if source.respond_to?(:encode)
+        source = ExecJS::encode(source) if source.respond_to?(:encode)
 
         if /\S/ =~ source
           unbox @rhino_context.eval("(#{source})")
diff --git a/lib/execjs/runtimes.rb b/lib/execjs/runtimes.rb
index cb4d837..52fd81c 100644
--- a/lib/execjs/runtimes.rb
+++ b/lib/execjs/runtimes.rb
@@ -90,4 +90,28 @@ module ExecJS
   def self.runtimes
     Runtimes.runtimes
   end
+
+  if defined? Encoding
+    if (!defined?(RUBY_ENGINE) || RUBY_ENGINE != "jruby")
+      def self.encode(string)
+        string.encode('UTF-8')
+      end
+    else
+      # workaround for jruby bug http://jira.codehaus.org/browse/JRUBY-6588
+      def self.encode(string)
+        if string.encoding.name == 'ASCII-8BIT'
+          data = string.dup
+          data.force_encoding('utf-8')
+
+          unless data.valid_encoding?
+            raise Encoding::UndefinedConversionError, "Could not encode ASCII-8BIT data #{string.dump} as UTF-8"
+          end
+        else
+          data = string.encode('utf-8')
+        end
+        data
+      end
+    end
+  end
+
 end