unexpected problem with postcircumfix:<[ ]>

$ ./perl6 t/spec/S32-str/encode.t
1..13
ok 1 - $str.encode returns a Buf
ok 2 - encoding to ASCII
ok 3 - right length of Buf
ok 4 - encoding to UTF-8
ok 5 - right length of Buf
get_pmc_keyed() not implemented in class ''
  in 'at_pos' at line 1056:CORE.setting
  in 'postcircumfix:<[ ]>' at line 1053:CORE.setting
  in main program body at line 1

$ git diff
diff --git a/src/core/Buf.pm b/src/core/Buf.pm
index 5d7d7d8..457e5ee 100644
--- a/src/core/Buf.pm
+++ b/src/core/Buf.pm
@@ -1,4 +1,4 @@
-role Buf[::T = Int] does Stringy {
+role Buf[::T = Int] does Stringy does Positional {
     has T @.contents;
 
     multi method new(@contents) {
@@ -34,6 +34,10 @@ role Buf[::T = Int] does Stringy {
     multi method elems() {
         @.contents.elems;
     }
+
+    multi method postcircumfix:<[ ]>(*@indices) {
+        @.contents[|@indices];
+    }
 }
 
 our multi sub infix:<eqv>(Buf $a, Buf $b) {