I have an .xslt which places an image fine...however, when no image is selected I get the above error: "Error parsing XSLT file:"

My code is now as below...can anyone help me with an if statement preventing the error when no image is selected in the media picker?

<!DOCTYPE xsl:stylesheet [ <!ENTITY nbsp "&#x00A0;"> ]>

<xsl:stylesheet 
   version="1.0" 
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform" 
 xmlns:msxml="urn:schemas-microsoft-com:xslt"
  xmlns:umbraco.library="urn:umbraco.library"
   exclude-result-prefixes="msxml umbraco.library">


<xsl:output method="xml" omit-xml-declaration="yes"/>

<xsl:param name="currentPage"/>


<xsl:variable name="imgSource" select="umbraco.library:GetMedia($currentPage/data[@alias='ProductLink1'], 'false')/data [@alias = 'umbracoFile']" />


<xsl:template match="/">

   
<img src="{$imgSource}" />   
 

</xsl:template>

</xsl:stylesheet>