diff --git a/Gemfile b/Gemfile
index fb1af24..16e9016 100644
--- a/Gemfile
+++ b/Gemfile
@@ -8,6 +8,6 @@ group :test do
   # see https://github.com/nu7hatch/mustang/issues/18
   gem 'mustang',      :platform => :ruby,
     :git => "https://github.com/nu7hatch/mustang.git", :ref => "2a3bcfbd9fd0f34e9b004fcd92188f326b40ec2a"
-  gem 'therubyracer', :platform => :ruby
+  gem 'therubyracer', :platform => :mri
   gem 'therubyrhino', :platform => :jruby
 end
diff --git a/execjs.gemspec b/execjs.gemspec
index 374c32a..e1e085c 100644
--- a/execjs.gemspec
+++ b/execjs.gemspec
@@ -14,16 +14,23 @@ Gem::Specification.new do |s|
   s.add_dependency "multi_json", "~>1.0"
   s.add_development_dependency "rake"
 
-  if RUBY_VERSION < "1.9" && (!defined?(RUBY_ENGINE) || RUBY_ENGINE == "ruby")
+  # mri_18  
+  if (!defined?(RUBY_ENGINE) || RUBY_ENGINE == "ruby") && RUBY_VERSION < "1.9"
     # see https://github.com/jbarnette/johnson/issues/21
     s.add_development_dependency "johnson"
   end
 
-  if (!defined?(RUBY_ENGINE) || RUBY_ENGINE != "jruby")
-    s.add_development_dependency "mustang"
+  # mri
+  if (!defined?(RUBY_ENGINE) || RUBY_ENGINE == "ruby")
+    # disabled for rbx, because of https://github.com/cowboyd/therubyracer/issues/157
     s.add_development_dependency "therubyracer"
-  else
+  end
+
+  # jruby
+  if (defined?(RUBY_ENGINE) && RUBY_ENGINE == "jruby")
     s.add_development_dependency "therubyrhino", ">=1.73.3"
+  else
+    s.add_development_dependency "mustang"
   end
 
   s.authors = ["Sam Stephenson", "Josh Peek"]