IronRuby Rake and -X:PrivateBrowsing

I’m trying to setup a Rake script to run all my IronRuby rspec tests.
The tests need to have access to internal constructors and methods.
IronRuby supports this with the -X:PrivateBinding switch on ir.exe.

So, for example, if my test is called some_tests.rb, the following
command works:
ir -X:PrivateBinding -S spec some_tests.rb

However, when I try to run rake that way it bombs out.

To repo, follow these steps:
Write a rakefile like the following:

task :test do

Running this command will succeed:
ir -S rake test

Running this command will fail:
ir -X:PrivateBinding -S rake test

with the following error: C:/IronRuby
1.1/lib/ironruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:1275: syntax
error, unexpected `.’

I’m running IronRuby on .NET 4.0.30319.1 with rake 0.8.7.

I also posted this question to Stack Overflow:

@ironshay suggested I post it here as well.