Unknown Runtime Error In IE, but not FF

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I use Ajax to update forms, so for instance:

 

<div class="formsleft">Do you want to buy apples?</div>
<div class="formsright">
<select name="buyapples" onchange="showField(this.value,'quantity')">
	<option value="No"<? if ($_POST['buyapples'] == "No") { echo ' selected="selected"'; } ?>>No</option>
             <option value="Yes"<? if ($_POST['buyapples'] == "Yes") { echo ' selected="selected"'; } ?>>Yes</option>
             </select>
</div>
<div id="quantity"></div>

 

So whatever I click, shows that option in the id.  Now my problem is if I want to do an onchange with the updated content like so:

 

<div class="formsleft">Do you want to buy apples?</div>
<div class="formsright">
<select name="buyapples" onchange="showField(this.value,'quantity')">
	<option value="No"<? if ($_POST['buyapples'] == "No") { echo ' selected="selected"'; } ?>>No</option>
             <option value="Yes"<? if ($_POST['buyapples'] == "Yes") { echo ' selected="selected"'; } ?>>Yes</option>
             </select>
</div>
<div id="quantity">
<div class="formsleft">How many apples do you want to buy?</div>
<div class="formsright">
<select name="applequantity" onchange="showField(this.value,'appletype')">
	<option value="1"<? if ($_POST['applequantity'] == "1") { echo ' selected="selected"'; } ?>>1</option>
             <option value="2"<? if ($_POST['applequantity'] == "2") { echo ' selected="selected"'; } ?>>2</option>
             </select>
</div>
</div>
<div id="appletype"></div>

 

When I choose yes to buy apples, the code shows in the quantity id.  Then when I choose how many, it loads content into the appletype id.  This works fine in Firefox, but throws an "Unknown Runtime Error" in IE7.  Any ideas why?

 

Thanks,

 

-JC