Guard-Uglify throws "only generation of JSON objects or arrays allowed"

Guard-Uglify throws "only generation of JSON objects or arrays allowed"

I'm trying to uglify javascript using guard but i get following errors:
ERROR - Guard::Uglify failed to achieve its , exception was: [#]
JSON::GeneratorError: only generation of JSON objects or arrays allowed
[#] C:/Ruby193/lib/ruby/1.9.1/json/common.rb:216:in generate' [#]
C:/Ruby193/lib/ruby/1.9.1/json/common.rb:216:ingenerate' [#]
C:/Ruby193/lib/ruby/1.9.1/json/common.rb:352:in dump' [#]
C:/Ruby193/lib/ruby/gems/1.9.1/gems/execjs-2.0.0/lib/execjs/external_runtime.rb:53:inblock
(2 levels) in compile' [#]
C:/Ruby193/lib/ruby/gems/1.9.1/gems/execjs-2.0.0/lib/execjs/external_runtime.rb:51:in
sub!' [#]
C:/Ruby193/lib/ruby/gems/1.9.1/gems/execjs-2.0.0/lib/execjs/external_runtime.rb:51:inblock
in compile' [#]
C:/Ruby193/lib/ruby/gems/1.9.1/gems/execjs-2.0.0/lib/execjs/external_runtime.rb:47:in
tap' [#]
C:/Ruby193/lib/ruby/gems/1.9.1/gems/execjs-2.0.0/lib/execjs/external_runtime.rb:47:incompile'
I'm using json 2.0.0(also tried with 1.8.0) and mutli-json 1.7.8(also
tried 1.7.9)