A sequence of more than one item is not allowed as the result of call to local: getFirstPublisherUrl

I'm trying to get the url of a publisher through an xQuery function, the function will pass the name of an author that I have in the biblio.xml document, and what I want is that given that name find the first pusblisher of a book written by that author. The information about the publisher's url is in the publist.xml file, this is the code of my function:

declare function local:getFirstPublisherUrl($aut as xs:string) as  xs:string 

{
for $b in doc("biblio.xml")/bib/book[author=$aut]/publisher[position()=1]
for $p in doc("publist.xml")/publist/publisher[pubname=$b]

return
    if($p[puburl])then
        $p/puburl/text()
     else
        " "
};

This is the call to my function:

<ebazpena ariketa="hamabi">
<puburlak>
{
for $aut in distinct-values(doc('biblio.xml')//author)

return 
    <puburl>
    { local:getFirstPublisherUrl(data($aut))  }
    </puburl>
   }
</puburlak>
</ebazpena>

This is my publist.xml file:

<?xml version="1.0" encoding="UTF-8"?>
<publist>
    <publisher>
        <pubname>Addison-Wesley</pubname>
        <pubaddress>Washington</pubaddress>
        <puburl>http://www.addisonwesley.com/index.html</puburl>
    </publisher>
    <publisher>
        <pubname>Morgan Kaufmann Publishers</pubname>
        <pubaddress>New York</pubaddress>
        <puburl>http://www.morgan.com/</puburl>
    </publisher>
    <publisher>
        <pubname>Wiley</pubname>
        <pubaddress>London</pubaddress>
        <puburl>http://www.wiley.uk/</puburl>
    </publisher>
    <publisher>
        <pubname>Springer-Verlag</pubname>
        <pubaddress>Berlin</pubaddress>
    </publisher>
</publist>

And this is the biblio.xml file:

<?xml version="1.0" encoding="UTF-8"?>
<bib>
    <book year="1994">
        <title>TCP/IP Illustrated</title>
        <author>
            <last>Stevens</last>
            <first>W.</first>
        </author>
        <publisher>Addison-Wesley</publisher>
        <price>65.95</price>
    </book>

    <book year="1992">
        <title>Advanced Programming in the Unix environment</title>
        <author>
            <last>Stevens</last>
            <first>W.</first>
        </author>
        <publisher>Addison-Wesley</publisher>
        <price>65.95</price>
    </book>
    <book year="2000">
        <title>Data on the Web</title>
        <author>
            <last>Abiteboul</last>
            <first>Serge</first>
        </author>
        <author>
            <last>Buneman</last>
            <first>Peter</first>
        </author>
        <author>
            <last>Suciu</last>
            <first>Dan</first>
        </author>
        <publisher>Morgan Kaufmann Publishers</publisher>
        <price>39.95</price>
    </book>
</bib>

The error is given to me in the line of the function:

for $b in doc("biblio.xml")/bib/book[author=$aut]/publisher[position()=1]

And it says the following:

  

A sequence of more than one item is not allowed as the result of a call   to local: getFirstPublisherUrl (" link ...",   " link ...")