diff --git a/lib/execjs/external_runtime.rb b/lib/execjs/external_runtime.rb
index 850fa4f..8a926ab 100644
--- a/lib/execjs/external_runtime.rb
+++ b/lib/execjs/external_runtime.rb
@@ -4,8 +4,10 @@ module ExecJS
   class ExternalRuntime
     class Context
       def initialize(runtime, source = "")
+        source = source.encode('UTF-8') if source.respond_to?(:encode)
+
         @runtime = runtime
-        @source  = source.respond_to?(:encode) ? source.encode("UTF-8") : source
+        @source  = source
       end
 
       def eval(source, options = {})
diff --git a/lib/execjs/mustang_runtime.rb b/lib/execjs/mustang_runtime.rb
index f6b8f86..31b3ce9 100644
--- a/lib/execjs/mustang_runtime.rb
+++ b/lib/execjs/mustang_runtime.rb
@@ -2,6 +2,8 @@ module ExecJS
   class MustangRuntime
     class Context
       def initialize(source = "")
+        source = source.encode('UTF-8') if source.respond_to?(:encode)
+
         @v8_context = ::Mustang::Context.new
         @v8_context.eval(source)
       end
diff --git a/lib/execjs/ruby_racer_runtime.rb b/lib/execjs/ruby_racer_runtime.rb
index db5582e..d6136ff 100644
--- a/lib/execjs/ruby_racer_runtime.rb
+++ b/lib/execjs/ruby_racer_runtime.rb
@@ -2,6 +2,8 @@ module ExecJS
   class RubyRacerRuntime
     class Context
       def initialize(source = "")
+        source = source.encode('UTF-8') if source.respond_to?(:encode)
+
         @v8_context = ::V8::Context.new
         @v8_context.eval(source)
       end
diff --git a/lib/execjs/ruby_rhino_runtime.rb b/lib/execjs/ruby_rhino_runtime.rb
index 0c5aa24..9f21371 100644
--- a/lib/execjs/ruby_rhino_runtime.rb
+++ b/lib/execjs/ruby_rhino_runtime.rb
@@ -2,6 +2,8 @@ module ExecJS
   class RubyRhinoRuntime
     class Context
       def initialize(source = "")
+        source = source.encode('UTF-8') if source.respond_to?(:encode)
+
         @rhino_context = ::Rhino::Context.new
         @rhino_context.eval(source)
       end
diff --git a/test/test_runtime.rb b/test/test_runtime.rb
index fa4052c..c25cfe5 100644
--- a/test/test_runtime.rb
+++ b/test/test_runtime.rb
@@ -69,7 +69,7 @@ class TestRuntime < Test::Unit::TestCase
 
       assert_raise Encoding::UndefinedConversionError do
         binary = "\xde\xad\xbe\xef".force_encoding("BINARY")
-        @runtime.eval(binary)
+        context.eval(binary)
       end
     end
   end