malloc error

$ cat produces-malloc-error 
Q:PIR {
    .local pmc list
    list = new 'ResizablePMCArray'
    push list, 195
    push list, 182

    .local pmc iterator
    iterator = iter list
    .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
};

$ perl6 produces-malloc-error 
perl6(77040) malloc: *** error for object 0x1056b5d70: pointer being reallocated was not allocated
*** set a breakpoint in malloc_error_break to debug
Abort trap