ruby on rails - gem update --system error: no implicit conversion of nil into String -

i'm running on windows , not able update rubygems either using command

gem update --system 

or manually installing gem. following error

error:  while executing gem ... (typeerror)     no implicit conversion of nil string 

i don't have issue while installing or updating other gems far can see.

this complete output when running --verbose

http://pasted.co/11325f4e

this bug in ruby gem installer system.

patch file installer.rb:

replace:

if ruby_executable       question << existing 

with:

if ruby_executable       question << (existing || 'an unknown executable')