'hello world' doesn't work

[steveklabnik]

$ cat Rakefile
<snip>
  config.source_files = "foo.md"
end
$ cat foo.md 
# Hello

world
$ rake
mkdir -p build
create build/spine.xhtml
rake aborted!
Don't know how to build task ''

Tasks: TOP => default => deliverables => build/deliverables/secret.pdf => build/master/prince_master.xhtml => build/master/master.xhtml => build/skeleton.xhtml => build/codex.xhtml => build/sections/foo.xhtml => build/exports/foo.html
(See full trace by running task with --trace)

😢