javascript 'tagName' is null or not an object in IE8 - Not validating radiobuttons in Ie8 - javascript-framework

Question

I am using validatious-custom-0.9.1.js framework in my project and I am trying to validate required fields for radio buttons. But it is not validating the radio buttons in IE8. I am getting the following error in console
Message: 'tagName' is null or not an object
Line: 7
Char: 9921
Code: 0
URI: http://validatious.org/design/js/validatious.0.9.1.min.js?1256063644
HTML code used.
<div>
<label for="_button_880">Radio List
<div>
<p>some text</p>
</div>
</label>
<div>
<ul>
<li>
<div>
<div>
<input type="radio" name="radio1" id="radio1" value="Radio1" class="required" title="Required."/>
<label for="radio1">Radio1</label>
</div>
</div>
</div>
</div>
JS where it is failing - First 2 are the examples that are given in the framework file. So you will get to know how it is implemented
/* A radio button field is assumed to be either a list - ordered or unordered -
with some element in front that acts as a label. This may be any element.
If it is not in a list (ie the element does not have "li" parent elements),
the label is assumed to be the element before the first input element.
Example1 (list approach):
<h2>Favourite food:</h2>
<ul>
<li>
<input type="radio" name="food" value="hamburger" id="food_hamburger" />
<label for="food_hamburger">Haburger</label>
</li>
<li>
<input type="radio" name="food" value="pizza" id="food_pizza" />
<label for="food_pizza">Pizza</label>
</li>
</ul>
getLabel() will in this case return the h2 element.
Example2 (no list).
<label class="groupLabel">Favourite food:</label>
<input type="radio" name="food" value="hamburger" id="food_hamburger" />
<label for="food_hamburger">Hamburger</label>
<input type="radio" name="food" value="pizza" id="food_pizza" />
<label for="food_pizza">Pizza</label>
getLabel() will in this case return the first label element
*/
Actual code
getLabel: function() {
var parent = this.__elements[0].parentNode;
//alert(parent1.nodeName); --- Gives 'nodeName' is null or not an object IN Ie8
if (parent1.tagName.toLowerCase() === 'li') { ---Gives 'tagName' is null or not an object
return v2.$(parent1.parentNode).previous();
}
var element = v2.$(this.__elements[0]).previous();
return element || this.base();
}
Required field validation in the js file
v.reg('required', function(field, value, params) {
return !v2.empty(value) && !(typeof value.length !== 'undefined' && value.length == 0);
}, null, null, 'not-empty', false);
It works fine in firefox, IE7 and IE9. But in Ie8, I get message tagName is null or not an object.
Can somebody please help me in this. If this is solved, then this framework will be very useful to avoid all the hardships in validating.
Thanks in advance..