vbScript function returns a value that cannot be converted to an XSL data type

I get the below error when I try to display XML data using XSL. The VBScript Function throws this error:

Function "MyFuctionName" returns a value that cannot be converted to an XSL data type. What could be the cause.
Snapshot of Code below: 1.XML

        <Client>
             <ClientID>34987886</ClientID>
               <LnkSeqNo>1</LnkSeqNo>
        </Client>

2.XSL code,function.

    Dim ClientID

    function SetClientID(ID)
        ClientID = ID
    end function

...

    <xsl:for-each select="Object/Client">
           <xsl:if test="LnkSeqNo[. = '1' or . = '57']">
              <xsl:value-of disable-output-escaping="yes" select="vbs:SetClientID(string(ClientID))"/>
..
          </xsl:if>
    </xsl:for-each>