Unable to bundle gem

[Vallantyn]

I have a very minimalist build_config.rb to test that:

MRuby::Build.new do |conf|
  if ENV['VisualStudioVersion'] || ENV['VSINSTALLDIR']
    toolchain :visualcpp
  else
    toolchain :gcc
  end

  enable_debug
  
  conf.gembox 'default'

  conf.gem 'path/to/mruby-gem_a'
  
  conf.gem :github => 'mattn/mruby-require'
  
  conf.gem 'path/to/mruby-gem_b'

Here mruby-gem_a and mruby-gem_b are just dummy names.
I tried using minirake and rake, the same problem arise:

Build summary:

================================================
      Config Name: host
 Output Directory: build/x86/host
         Binaries: mrbc
    Included Gems:
             mruby-dir
             mruby-io - IO and File class
             mruby-random - Random class
             mruby-sprintf - standard Kernel#sprintf method
             mruby-time - standard Time class
             mruby-tempfile
             mruby-pack - Array#pack and String#unpack method
             mruby-print - standard print/puts/p
             mruby-math - standard Math module
             mruby-struct - standard Struct class
             mruby-compar-ext - Enumerable module extension
             mruby-enum-ext - Enumerable module extension
             mruby-string-ext - String class extension
             mruby-numeric-ext - Numeric class extension
             mruby-array-ext - Array class extension
             mruby-hash-ext - Hash class extension
             mruby-range-ext - Range class extension
             mruby-proc-ext - Proc class extension
             mruby-symbol-ext - Symbol class extension
             mruby-object-ext - Object class extension
             mruby-objectspace - ObjectSpace class
             mruby-fiber - Fiber class
             mruby-enumerator - Enumerator class
             mruby-enum-lazy - Enumerator::Lazy class
             mruby-toplevel-ext - toplevel object (main) methods extension
             mruby-compiler - mruby compiler library
             mruby-bin-mirb - mirb command
               - Binaries: mirb
             mruby-error - extensional error handling
             mruby-bin-mruby - mruby command
               - Binaries: mruby
             mruby-bin-strip - irep dump debug section remover command
               - Binaries: mruby-strip
             mruby-kernel-ext - Kernel module extension
             mruby-class-ext - class/module extension
             mruby-gem_a
             mruby-bin-mrbc - mruby compiler executable
             mruby-require
================================================

** Invoke C:/Workspace/MRuby/build/x86/host/lib/mruby-gem_b.so (first_time)
rake aborted!
Don't know how to build task 'C:/Workspace/MRuby/build/x86/host/mrbgems/mruby-gem_b/src/mruby-gem_b.obj  (see --tasks)

@pbosetti you made the change about that feature, right ? What am I missing here ?