Problem documentation and/or compiler

[mt1957]

L.s.

At http://doc.perl6.org/type/Str#method_encode it says that string 
encoding returns a Buf.
It returns a utf8 instead see repl session below.
> my Str $s = 'abcdef';
abcdef
> my Buf $b = $s.encode;
Type check failed in assignment to '$b'; expected 'Buf' but got 'utf8'
  in block <unit> at <unknown file>:1


Marcel