[MarkLogic Dev General] update function

[Eric Palmitesta]

I'm putting together a function which updates a single element in an xml 
file, however I'm getting an error I'm not sure how to deal with.

(WARNING, INCOMING EVAL)

define function update($uri as xs:string, $path as xs:string, $value as 
item())
{
   let $old-node := xdmp:eval(concat('doc("', $uri, '")', $path))
   let $new-node := element { tokenize($path, "/")[last()] } { text { 
$value } }
   return
     xdmp:node-replace($new-node, $old-node)
}

When I run something like:

update('/path/to/file.xml', '/path/to/element', 'new value')

$old-node should be doc("/path/to/file.xml")/path/to/element
$new-node should be <element>new value</element>

I get:

XDMP-UPCONSTNODES: Cannot update constructed nodes


What's strange is if I replace the $new-node and $old-node variables 
with their assigned values above, that same call gets me:

XDMP-UPEXTNODES: Cannot update external nodes

What am I missing here?

Eric