lastIndexOf does not work in Internet Explorer

I have the following code:

var currentServerlist = [];
    var newServerIp = document.getElementById('add_server').value; 
    if( CurrentServerIP != newServerIp )
    {
        $('#failoverServers td.row_selector').each(function() {
        var row = $(this).closest('tr');
        var serverIp = row.find('td[rel=ip]').text();
        currentServerlist.push(serverIp);	
        });

        if(currentServerlist.lastIndexOf(newServerIp) != -1)
        {
            return true;
        }
        return false;

Select allOpen in new window

But I find that the lastIndexOf does not work in InternetExplorer (It does in Chrome).

How can I fix this?