[jQuery] xml parse error onreadystatechange

I'm confused....why is the xml parse error being thrown when I'm
dealing with json?

if ( status == "success" ) {
// Watch for, and catch, XML document parse errors
try {
// process the data (runs the xml through httpData regardless of
callback)
data = jQuery.httpData( xml, s.dataType );
} catch(e) {
status = "parsererror";
}

The call is:

$.ajax({ type: "POST", contentType: "application/json; charset=utf-8",
url: "getColors.aspx",
data: "{}",
dataType: "json",
success: function(data) {
$(dropdownSet).loadSelect(data);
adjustSizeDropdown();
},
error: function(xhr) {
$('#errorDisplay').html('Error: ' + xhr.status + ' '
+ xhr.statusText);
$('#errorDisplay').css('display', 'block');
} });