the Buf code that crashes

$ cat src/core/Buf.pm 
role Buf[::T = Int] does Stringy {
    has T @.contents;

    multi method new(@contents) {
        self.bless(*, :contents(@contents.list));
    }

    multi method decode($encoding = 'UTF-8') {
        my @contents = @.contents;
        my $str = ~Q:PIR {
            $P0 = find_lex '@contents'

            .local pmc bb
            .local string s
            bb = new ['ByteBuffer']
            bb[0] = 0xC3
            bb[1] = 0x91
            s = bb.'get_string_as'(utf8:unicode:"")
            %r = s

#            iterator = iter $P0
#            .local pmc sb
#            sb = new 'StringBuilder'
#            sb = unicode:""
#            loop:
#              unless iterator goto done
#              $P1 = shift iterator
#              $I1 = $P1
#              $S1 = chr $I1
#              sb .= $S1
#              goto loop
#            done:
#              %r = sb
        };
        return $str;
    }

    multi method elems() {
        @.contents.elems;
    }
}

our multi sub infix:<eqv>(Buf $a, Buf $b) {
    return $a.contents ~~ $b.contents;
}

$ ./perl6 t/spec/S32-str/encode.rakudo 1..10
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
ok 6 - # SKIP We do not handle NDF yet
Null PMC access in set_string_native()
  in 'decode' at line 3972:CORE.setting
  in main program body at line 21:t/spec/S32-str/encode.rakudo