My jQuery/Javascript knowledge is limited I'm afraid. I have a "how did you hear about us" dropdown on a form. However, I get the following Javascript error on change:

Error: '$viewMap[...]' is null or not an object

My dropdown looks like this:

<select onchange="setSourceID(this.value)" name="sourceID" id="sourceID" class="required">
    <option value="" selected="selected">Please choose&#8230;</option>
    <option value="National Paper">National Paper</option>
    <option value="Magazine">Magazine</option>
    <option value="Regional Paper">Regional Paper</option>
    <option value="9682">Internet Search</option>
    <option value="9684">Recommendation</option>
    <option value="9683">Other</option>
</select>

<!-- some additional dropdowns below that appear based on what's selected above -->
<select onchange="setSourceID(this.value)" name="referrerName[]" id="referrer1" class="smartField">
    <option value="" selected="selected">Please choose&#8230;</option>
<option value="The Times">The Times</option>
etc...
</select>

and so on...

My Javascript looks like this:

$(document).ready(function() {
    $('.smartField').hide();
    $.viewMap = {
    	'' : $([]),
    	'National Paper' : $('#referrer1'),
        'Magazine' : $('#referrer2'),
        'Regional Paper' : $('#referrer3')
    	//'Internet Search' : $('#referrer4'), 
    	//'Recommendation' : $('#referrer5'), 
    	//'Other' : $('#referrer6')
    };

    $("#sourceID").bind(($.browser.msie ? "click" : "change"), function () {
    	$.each($.viewMap, function() { this.hide(); }); // hide all
        $.viewMap[$(this).val()].show(); // show current
    });
});

Does anybody have any idea where I'm going wrong? Any help very much appreciated.