Error in jruby 1.7.0preview2 but not in preview1

jruby_error.pry

Using jruby 1.7.0preview2 I try to use IMGKit to create a file, I get:
    
    ╰─$ rails c
    Loading development environment (Rails 3.2.3)
    [1] pry(main)> kit = IMGKit.new("hello")
    #<IMGKit:0x0000683c
        attr_accessor :source = #<IMGKit::Source:0x00006840
            @source = "hello"
        >,
        attr_accessor :stylesheets = [],
        attr_reader :options = {}
    >
    [2] pry(main)> file = kit.to_file('test.jpg')
    Encoding::UndefinedConversionError: Input length = 1
    from org/jruby/RubyIO.java:1400:in `write'
    [3] pry(main)> 

Using jruby1.7.0preview1 It creates the file fine