Autocomplete hitting enter vs clicking different behaviour on IE7 (jQuery 1.4.3 + jQuery UI 1.8.5)

Question

here is my code:

$(document).ready(function() {
    $("#OperatorID").focus();
    $("#PartNumberID").autocomplete({
    source: "ajax/getData.php",
    minLength: 6,
    delay: 100,
    select: function(event, ui) {
            $("#PartNumberID").val(ui.item.value); 
            $("#boxID").val(ui.item.box); 
            $("#PackageID").val(ui.item.pkg);
    },
    change: function(event, ui){
            $.get('ajax/getProcess.php?box='+ui.item.box, function(result){
                $("#ProcessID").val(result);
      });
        }
    }); 
});

It works like its supposed to on IE8 and Firefox, however on IE7 if you start typing a part number and highlight the result using the keyboard and then hit enter to select, it selects it but then immediately submits the form, if you click on the result then it performs as expected (doesn't submit the form) except it doesn't automatically fill in ProcessID (nothing happens). I'm using jQuery 1.4.3 + jQuery UI 1.8.5 does anyone know how to make the behaviour work the same on IE7+8+firefox? (i.e click or hit enter on the autocomplete and have it fill in all those values and process without automatically submitting the form)

on IE7 when it submits the form automatically I also get this error:

'item.box' is null or not an object