XSL - Attribute and namespace nodes cannot be added to the parent element after a text, comment, pi, or sub-element node has already been added. Problem

I'm using ThumbGen and I get the Error: "Attribute and namespace nodes cannot be added to the parent element after a text, comment, pi, or sub-element node has already been added."

Here is the XSLT file:
<?xml version="1.0" encoding="UTF-8"?>

    <xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>

    <xsl:variable name="pathvalue" select="//tokens/token[@name='%PATH%']"/>
    <xsl:variable name="folderactors" select="string('\..\Common\cast\')"/>
    <xsl:variable name="actorsext" select="string('.jpg')"/>
    <xsl:variable name="actorstext" select="string('')"/>
    <xsl:variable name="lista1" select="//tokens/token[@name='%ACTORS%']"/>
    <xsl:variable name="actor1" select="substring-before($lista1, '/')" />  
    <xsl:variable name="resto1" select="substring-after($lista1, '/')" />
    <xsl:variable name="lista2" select="$resto1"/>
    <xsl:variable name="actor2" select="substring-before($lista2, '/')" />      
<xsl:template match="ImageElement[@Name='Actor1']">
        <xsl:copy>
            <xsl:apply-templates select="@*"/>
            <xsl:choose>
                <xsl:when test="$actor1 != '' ">
                    <xsl:attribute name="SourceData">
                    <xsl:value-of select="concat($pathvalue,$folderactors,$actor1,$actorsext)"/></xsl:attribute>
                </xsl:when>
            </xsl:choose>
            <xsl:apply-templates select="node()"/>
        </xsl:copy>
    </xsl:template>

</xsl:stylesheet>


And here is the line in the XML that references it:
<ImageElement Name="Actor1" X="980" Y="44" Width="93" Height="123" Source="File" Offset="0" MultiPageIndex="-1" SourceData="" NullImageUrl="%PATH%\..\Common\cast\NoPhotoAvailable.jpg"></ImageElement>   


Can anyone tell me how to fix it?