How can I return JSONP in RestXQ (using eXist-db)?

I cannot figure out how to return JSONP in RestXQ. After adding let $x := util:declare-option("exist:serialize", fn:concat("method=json jsonp=",request:get-parameter("callback", "callback"))) to the function, I get the error message:

err:XPTY0004:It is a type error if, during the static analysis phase, an expression is found to have a static type that is not appropriate for the context in which the expression occurs, or during the dynamic evaluation phase, the dynamic type of a value does not match a required type as specified by the matching rules in 2.5.4 SequenceType Matching.

The beginning of the GET function is:

declare
    %rest:GET
    %rest:path("/demo/contacts/submit")
    %rest:query-param("email", "{$email}", '')     
    %rest:query-param("nomail", "{$nomail}", 0)    
    %rest:produces("application/javascript")
    %output:media-type("application/javascript")
    %output:method("json")
function contacts:submit($email as xs:string*, $nomail as xs:integer*)
{


    try
    {

        let $x := util:declare-option("exist:serialize", fn:concat("method=json jsonp=",request:get-parameter("callback", "callback")))

As discussed on the eXist-open mailing list (I'd suggest joining!), the request module's get-parameter() function is not available inside a RestXQ function. Instead, you can get your callback parameter via the %rest:query-param annotation. Add %rest:query-param("callback", "{$callback}", '') to your contacts:submit() function, and I think you'll be a step closer.