XUpate exception in september snapshots

Hi, I am experiencing a problem with the two september snapshots: I
have a query that also performs an XUptade. Its goal is to generate
unique numbers that I use for putting ids to xml nodes.
I have an ids.xml file that contains something like: <ids>100</ids>.
And here is the request:
xquery version "1.0";
declare namespace xmldb="http://exist-db.org/xquery/xmldb";;
let $root := xmldb:collection('xmldb:exist:///db/rep1', 'admin', '')
let $id := document('/db/rep1/ids.xml')/ids 
let $mods := xmldb:update(
        $root,
        <xu:modifications version="1.0" xmlns:xu="http://www.xmldb.org/xupdate";>
                <xu:update select="document('/db/rep1/ids.xml')/ids">
                        {document('/db/rep1/ids.xml')/ids/number()+1}
                </xu:update>
        </xu:modifications>)
return <result id="{$id}"/>

Executing this request in any of the september snapshots throws the
below exception.
I also would like to know if there is a better way to generate unique
ids, I mean something automatic and thread-safe as sequences in SQL?

Here goes the exception

java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
        at java.util.LinkedList.entry(LinkedList.java:368)
        at java.util.LinkedList.get(LinkedList.java:313)
        at org.exist.xquery.PathExpr.getExpression(PathExpr.java:162)
        at org.exist.xquery.Function.getArgument(Function.java:293)
        at org.exist.xquery.functions.FunNumber.pprint(FunNumber.java:74)
        at org.exist.xquery.PathExpr.pprint(PathExpr.java:181)
        at org.exist.xquery.OpNumeric.pprint(OpNumeric.java:185)
        at org.exist.xquery.PathExpr.pprint(PathExpr.java:181)
        at org.exist.xquery.EnclosedExpr.pprint(EnclosedExpr.java:106)
        at org.exist.xquery.PathExpr.pprint(PathExpr.java:181)
        at 
org.exist.xquery.ElementConstructor.pprint(ElementConstructor.java:183)
        at org.exist.xquery.PathExpr.pprint(PathExpr.java:181)
        at 
org.exist.xquery.ElementConstructor.pprint(ElementConstructor.java:183)
        at org.exist.xquery.PathExpr.pprint(PathExpr.java:181)
        at org.exist.xquery.Function.pprint(Function.java:336)
        at org.exist.xquery.PathExpr.pprint(PathExpr.java:181)
        at org.exist.xquery.LetExpr.pprint(LetExpr.java:119)
        at org.exist.xquery.LetExpr.pprint(LetExpr.java:129)
        at org.exist.xquery.LetExpr.pprint(LetExpr.java:129)
        at org.exist.xquery.PathExpr.pprint(PathExpr.java:181)
        at org.exist.xquery.XQuery.compile(XQuery.java:94)
        at org.exist.xquery.XQuery.compile(XQuery.java:69)
        at org.exist.xmlrpc.RpcConnection.doQuery(RpcConnection.java:256)
        at org.exist.xmlrpc.RpcConnection.queryP(RpcConnection.java:1237)
        at org.exist.xmlrpc.RpcServer.queryP(RpcServer.java:794)
        at org.exist.xmlrpc.RpcServer.queryP(RpcServer.java:774)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at 
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at 
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:585)
        at 
org.exist.xmlrpc.AuthenticatedHandler.execute(AuthenticatedHandler.java:116)
        at 
org.exist.xmlrpc.AuthenticatedHandler.execute(AuthenticatedHandler.java:63)
        at org.apache.xmlrpc.XmlRpcWorker.invokeHandler(Unknown Source)
        at org.apache.xmlrpc.XmlRpcWorker.execute(Unknown Source)
        at org.apache.xmlrpc.XmlRpcServer.execute(Unknown Source)
        at org.apache.xmlrpc.XmlRpcServer.execute(Unknown Source)
        at org.exist.xmlrpc.RpcServlet.doPost(RpcServlet.java:39)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:760)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
        at 
org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:366)
        at 
org.mortbay.jetty.servlet.WebApplicationHandler.dispatch(WebApplicationHandler.java:293)
        at 
org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:581)
        at org.mortbay.http.HttpContext.handle(HttpContext.java:1687)
        at 
org.mortbay.jetty.servlet.WebApplicationContext.handle(WebApplicationContext.java:544)
        at org.mortbay.http.HttpContext.handle(HttpContext.java:1637)
        at org.mortbay.http.HttpServer.service(HttpServer.java:875)
        at org.mortbay.http.HttpConnection.service(HttpConnection.java:806)
        at org.mortbay.http.HttpConnection.handleNext(HttpConnection.java:956)
        at org.mortbay.http.HttpConnection.handle(HttpConnection.java:823)
        at 
org.mortbay.http.SocketListener.handleConnection(SocketListener.java:202)
        at org.mortbay.util.ThreadedServer.handle(ThreadedServer.java:290)
        at org.mortbay.util.ThreadPool$JobRunner.run(ThreadPool.java:743)
        at java.lang.Thread.run(Thread.java:595)


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