Smarty как отловить и залечить ошибку?

 
[Fri Jun 19 17:32:29 2015] [error] [client 78.85.52.9] PHP Warning:  Smarty error: math: parameter x is empty in  .../includes/smarty/libs/Smarty.class.php on line 1095, referer: http://сайт.ru/.../bolee-500-zhitelei.html
 

 
<ul id="con_pt_list">
				{foreach key=tid item=pages from=$pt_pages}
					{if ($tid+1 != $page)}
						{math equation="x + 1" x=$tid assign="key"}
						<li><a href="{$pages.url}">{$pages.title}</a></li>
					{else}
						<li>{$pages.title}</li>
					{/if}
				{/foreach}
				<ul>
 

 
<ul id="con_pt_list">
				{foreach key=tid item=pages from=$pt_pages}
					{if ($tid+1 != $page)}
						{$tid + 1 assign="key"}
						<li><a href="{$pages.url}">{$pages.title}</a></li>
					{else}
						<li>{$pages.title}</li>
					{/if}
				{/foreach}
				<ul>
 

 
[Fri Jun 19 20:22:32 2015] [error] [client 37.9.118.28] PHP Warning:  Smarty error: math: parameter x is empty in .../includes/smarty/libs/Smarty.class.php on line 1095
 

{$tid + 1 assign="key"}

{$key = $tid + 1}

 
[Tue Jun 23 14:31:24 2015] [error] [client 217.69.134.63] PHP Warning:  Smarty error: math: parameter x is empty in .../includes/smarty/libs/Smarty.class.php on line 1095