unable to get property 'showmodaldialog' of undefined or null reference

I got "unable to get property 'showmodaldialog' of undefined or null reference" error when I try to open the list displayform in SharePoint 2013 modal dialog.

This is my script code:

<script type="text/javascript">
        function openDialog( pUrl ) {  
         SP.UI.ModalDialog.showModalDialog(   
         {  
          url: pUrl,
         width: 650,  
         height:600,  
         title: "MyTitle"
          }  
         );  
    }  
</script>

calling:

<a href="javascript:openDialog('../../News/Lists/Posts/Post.aspx?ID={@ows_ID}')"><xsl:value-of select="@ows_LinkTitle"/></a>

could anyone help me to resolve this issue?