RbReadline.rl_insert_text can not insert Chinese characters

[fffx]

My enviroments: rb-readline (0.5.5), ruby 2.2.2
In my pry console, when try to insert Chinese characters.
I typed, and hit Enter

RbReadline.rl_insert_text("我的".)

It becomes

> RbReadline.rl_insert_text("������")
=> 6

If I add call force_encoding("utf-8") then, I got a Encoding::CompatibilityError error:

> RbReadline.rl_insert_text("������".force_encoding("utf-8"))
Encoding::CompatibilityError: incompatible character encodings: ASCII-8BIT and UTF-8
from /home/fangxing/.rvm/gems/ruby-2.2.2@global/gems/rb-readline-0.5.5/lib/rbreadline.rb:4905:in `[]='
> RbReadline.rl_insert_text("".force_encoding("utf-8"))Error: incompatible character encodings: ASCII-8BIT and UTF-8
/home/fangxing/.rvm/gems/ruby-2.2.2@global/gems/rb-readline-0.5.5/lib/rbreadline.rb:3494:in `[]='
/home/fangxing/.rvm/gems/ruby-2.2.2@global/gems/rb-readline-0.5.5/lib/rbreadline.rb:3494:in `rl_redisplay'
/home/fangxing/.rvm/gems/ruby-2.2.2@global/gems/rb-readline-0.5.5/lib/rbreadline.rb:4728:in `_rl_internal_char_cleanup'
/home/fangxing/.rvm/gems/ruby-2.2.2@global/gems/rb-readline-0.5.5/lib/rbreadline.rb:4789:in `readline_internal_charloop'
/home/fangxing/.rvm/gems/ruby-2.2.2@global/gems/rb-readline-0.5.5/lib/rbreadline.rb:4853:in `readline_internal'
/home/fangxing/.rvm/gems/ruby-2.2.2@global/gems/rb-readline-0.5.5/lib/rbreadline.rb:4875:in `readline'
/home/fangxing/.rvm/gems/ruby-2.2.2@global/gems/rb-readline-0.5.5/lib/readline.rb:45:in `readline'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/repl.rb:199:in `block in input_readline'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/input_lock.rb:115:in `call'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/input_lock.rb:115:in `interruptible_region'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/repl.rb:198:in `input_readline'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/repl.rb:184:in `block in read_line'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/repl.rb:129:in `handle_read_errors'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/repl.rb:170:in `read_line'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/repl.rb:98:in `read'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/repl.rb:68:in `block in repl'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/repl.rb:67:in `loop'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/repl.rb:67:in `repl'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/repl.rb:38:in `block in start'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/input_lock.rb:61:in `call'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/input_lock.rb:61:in `__with_ownership'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/input_lock.rb:79:in `with_ownership'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/repl.rb:38:in `start'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/repl.rb:13:in `start'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/pry_class.rb:192:in `start'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-byebug-3.5.0/lib/pry-byebug/pry_ext.rb:11:in `start_with_pry_byebug'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/lib/pry/cli.rb:116:in `start'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/gems/pry-0.11.2/bin/pry:12:in `<top (required)>'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/bin/pry:23:in `load'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/bin/pry:23:in `<main>'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/bin/ruby_executable_hooks:15:in `eval'
/home/fangxing/.rvm/gems/ruby-2.2.2@rails4/bin/ruby_executable_hooks:15:in `<main>'

It seems RbReadline asset my string is in ASCII-8BIT, how do I tell him that is a utf-8 string ?