File.expand_path with tilde

Running this command in JRuby:
File.expand_path(File.join('~', 'some_file'))
=> "//some_file"
yields a return value that is not recognized by Windows. The same command in MRI:
=> "/some_file"
works fine.

I have noticed that using:
File.expand_path('some_file', '~')
=> "C:/some_file"
yields an acceptable result, although still different than the MRI result:
=> "/some_file"