diff --git a/Gemfile b/Gemfile
index a602f48..c65d6eb 100644
--- a/Gemfile
+++ b/Gemfile
@@ -14,5 +14,5 @@ group :test do
   else
     gem 'therubyrhino_jar', :path => '.'
   end
-  gem 'less', '>= 2.2.1'
+  gem 'less', '>= 2.2.1', :require => nil
 end
\ No newline at end of file
diff --git a/lib/rhino/error.rb b/lib/rhino/error.rb
index 847fbff..a77cd07 100644
--- a/lib/rhino/error.rb
+++ b/lib/rhino/error.rb
@@ -25,7 +25,7 @@ module Rhino
     # #Rhino::JS::JavaScriptException instance.
     def cause
       return @cause if defined?(@cause)
-      if @native.respond_to?(:cause)
+      if @native.respond_to?(:cause) && @native.cause
         @native.cause
       else
         @native.is_a?(JS::RhinoException) ? @native : nil