undefined array key 'ts' on line 57

<?php

function include_mostrecent()
{
    global $nen, $prj;
    $ret = '';

    $me_long = 'most recent';
    $me = str_replace( ' ', '', $me_long );

    $ext_array = array( '.7z', '.zip' );
    $ext_pcre = str_replace( '.', '\\.', implode( '|', $ext_array ) );

    $toc = fasttoc( 'showcase/vault/' . $prj . '/builds', $ext_array );

    if( count( $toc ) === 0 )
    {
        $nen['swap.ct'][$me] = count( $toc );
        $ret = '';
        return $ret;
    }

    $ct = 0;

    $span = 4;
    $count = 0;

    $pattern = '/(?<project>.*)(?<ts>(?<yyyy>[0-9]{4})-(?<mmdd>[0-9]{4})-(?<hhmm>[0-9]{4}))(?<comment>.*)(?<ext>\.(7x|zip))/';

    $latest = '0000-0000-0000';
    $latest_filename = '';

    $index = 0;
    for( $index = 0; $index < count( $toc ); $index++ )
    {
        preg_match( $pattern, $toc[ $index ], $matches );
        $timestamp = $matches['ts'];

        if( $timestamp > $latest )
        {
            $latest = $timestamp;
            $latest_filename = $toc[ $index ];
        }

    }

    $ret .= '<tr><td colspan="1" style="width:25%;text-align:center;font-size:170%;bold;">';

    $ret .= '<h2><a href="./showcase/vault/' . $prj . '/builds/' . $latest_filename . '">' . $latest_filename . '</a></h2>';

    $ret .= '</td>';
    $ret .= '</tr>';

    if ( strlen( $ret ) !== 0)
    {
        $ret = '<table colspan="' . $span . '" style="width:100%;"><th colspan="' . $span . '"><h2 style="text-align:center;" id="' . $me . '">' . ucwords( $prj ) . ' :: ' . ucwords($me_long) . '</h2></th>' . $ret . '</table>';
    }

    $nen[ 'swap.ct' ][ $me ] = count($toc);

    return $ret;
}

?>