assertion failure: "false && "nextImpl() called past iterator end"

Triggered by the following query

import module namespace http = "http://www.zorba-xquery.com/modules/http-client";

declare function local:data-guide($x as element()) as element()
{
  local:collapse-nodes($x, 0)
};

declare function
local:collapse-nodes($x as node()*, $id as xs:integer) as node()*
{
  for $y in $x
  let $n := node-name($y)
  group by $n
  count $i
  return local:collapse-nodes-same-name($y, $id+$i)
};

declare function
local:collapse-nodes-same-name($x as node()+, $id as xs:integer) as node()
{
  let $f:= $x[1]
  let $n :=node-name($f)
  return
    if ($f instance of element())
    then
      element {$n} { local:collapse-nodes($x/@*, $id+1), $id+2, local:collapse-nodes($x/*, $id+3) }
   else
      attribute {$n} {$id}
};

local:data-guide(http:get("http://28william.s3.amazonaws.com/books.xml")[2]/*)