yql-problem

I am trying to use yql   with xpath and its not working please advice me

[code]
SELECT *  FROM xml WHERE
url="http://gdata.youtube.com/feeds/api/videos?fields=entry(media:group(media:title,media:player,yt:duration))&max-results=2"
[/code]

returns
[code]
<?xml version="1.0" encoding="UTF-8"?>
<query xmlns:yahoo="http://www.yahooapis.com/v1/base.rng"
    yahoo:count="1" yahoo:created="2011-09-17T02:42:07Z" yahoo:lang="en-US">
    <diagnostics>
        <publiclyCallable>true</publiclyCallable>
        <url execution-start-time="1" execution-stop-time="329"
            execution-time="328" proxy="DEFAULT"><![CDATA[http://gdata.youtube.com/feeds/api/videos?fields=entry(media:group(media:title,media:player,yt:duration))&max-results=2]]></url>
        <user-time>330</user-time>
        <service-time>328</service-time>
        <build-version>21440</build-version>
    </diagnostics>
    <results>
        <feed xmlns="http://www.w3.org/2005/Atom"
            xmlns:media="http://search.yahoo.com/mrss/" xmlns:yt="http://gdata.youtube.com/schemas/2007">
            <entry>
                <media:group>
                    <media:player url="http://www.youtube.com/watch?v=3071VQtKEAE&amp;feature=youtube_gdata_player"/>
                    <media:title type="plain">Minecraft - "Shadow of Israphel" Part 16: A Beacon of Hope</media:title>
                    <yt:duration seconds="1368"/>
                </media:group>
            </entry>
            <entry>
                <media:group>
                    <media:player url="http://www.youtube.com/watch?v=ndVVrZqHqA8&amp;feature=youtube_gdata_player"/>
                    <media:title type="plain">Minecraft - "Better Than Wolves" Mod Spotlight</media:title>
                    <yt:duration seconds="1681"/>
                </media:group>
            </entry>
        </feed>
    </results>
</query>
[/code]


I wan to  add a xpath filter  to select on the entry whihc has a duration seconds 1681


so I added

[code]
SELECT *  FROM xml WHERE
url="http://gdata.youtube.com/feeds/api/videos?fields=entry(media:group(media:title,media:player,yt:duration))&max-results=2" and xpath='//yt:duration[@seconds=1681]'
[/code]

this return
[code]
<?xml version="1.0" encoding="UTF-8"?>
<query xmlns:yahoo="http://www.yahooapis.com/v1/base.rng"
    yahoo:count="0" yahoo:created="2011-09-17T02:43:51Z" yahoo:lang="en-US">
    <results/>
</query>
[/code]

I dont understand why  xpath filer is not working please advice  me