Magento – Ajax Error in system.log checkout/cart/sidebar/default.phtml on line 15

Today we had a template in which there were entries in the error log with the message:

Notice: Undefined index: isAjax in /PFAD/www/app/design/frontend/default/default/[UnserTemplate]/checkout/cart/sidebar/default.phtml on line 15

In this line of the above. File we found the following code:

			if($params['IsAjax'])

This was embedded in a function, which looked so:

        <a href="<?php 
	        $params = $this->getRequest()->getParams();
			if($params['IsAjax']) 
				$isAjax = 1;
			else
				$isAjax = 0;
			if($isAjax == 1){
				$refererUrl = Mage::registry('referrer_url');
		        if (empty($refererUrl)) {
		            $refererUrl = Mage::getBaseUrl();
		        }
				$deleteUrl = $this->getUrl(
		            'ajax/index/delete',
		            array(
		                Id =>$_item->getId(),
		                Mage_Core_Controller_Front_Action::PARAM_NAME_URL_ENCODED => $this->helper('core/url')->getEncodedUrl($refererUrl)
		            )
		        );

There were probably a few issues with the Ajax script. We have the o.g. Part then completely (of <a href bis ); ) replaced by the following code.

        <a href="<?php 
            $params = $this->getRequest()->getParams();
            if(isset($params['IsAjax'])) {
                if($params['IsAjax'] == 1){
                    $refererUrl = Mage::registry('referrer_url');
                    if (empty($refererUrl)) {
                        $refererUrl = Mage::getBaseUrl();
                    }
                    $deleteUrl = $this->getUrl(
                        'ajax/index/delete',
                        array(
                            Id =>$_item->getId(),
                            Mage_Core_Controller_Front_Action::PARAM_NAME_URL_ENCODED => $this->helper('core/url')->getEncodedUrl($refererUrl)
                        )
                    );
                }

Now the checkout works fine and there are no more error messages.

Used in Magento Version 1.7.0.2. Ask, Suggestions? Gladly!