I can return the numeric value of the dropdown 'articleType'.  I cant seem to get the text value using GetPreValueAsString. Thows an error when I add the line in italic containing the function. Tried ignoring errors but it wont parse.

<tr>

  <td class="content-kb-title"><a href="{umbraco.library:NiceUrl(@id)}"><xsl:value-of select="@nodeName"/></a></td>
  <td class="content-kb-doctype">
    <xsl:if test="./articleType &gt; 0">
      <xsl:value-of select="./articleType"/>
      <xsl:value-of select="umbraco.library:GetPreValueAsString(./articleType)"/>
    </xsl:if>
    </td>
  <td class="content-kb-lastdate"><xsl:value-of select="umbraco.library:FormatDateTime(@updateDate, 'd')"/>
</td>
  </tr>

Error occured

System.Xml.Xsl.XslTransformException: Extension functions cannot return null values. 
at System.Xml.Xsl.Runtime.XmlQueryRuntime.ChangeTypeXsltResult(XmlQueryType xmlType, Object value) 
at System.Xml.Xsl.Runtime.XmlQueryContext.InvokeXsltLateBoundFunction(String name, String namespaceUri, IList`1[] args) 
at (XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime) 
at Root(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime) 
at Execute(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime) 
at System.Xml.Xsl.XmlILCommand.Execute(Object defaultDocument, XmlResolver dataSources, XsltArgumentList argumentList, XmlSequenceWriter results) 
at System.Xml.Xsl.XmlILCommand.Execute(Object defaultDocument, XmlResolver dataSources, XsltArgumentList argumentList, XmlWriter writer) 
at System.Xml.Xsl.XslCompiledTransform.Transform(IXPathNavigable input, XsltArgumentList arguments, XmlWriter results, XmlResolver documentResolver) 
at System.Xml.Xsl.XslCompiledTransform.Transform(IXPathNavigable input, XsltArgumentList arguments, TextWriter results) 
at umbraco.presentation.webservices.codeEditorSave.SaveXslt(String fileName, String oldName, String fileContents, Boolean ignoreDebugging)