Woocommerce: Add product out of stock badge

functions.php

add_action('woocommerce_before_shop_loop_item_title','product_out_of_stock_badge');
function product_out_of_stock_badge() {
	global $woocommerce, $product, $post;
  $post_id = $post->ID;
	$available_variations = $product->get_available_variations();
    foreach ($available_variations as $variation) {
    	$isinstock = $variation[is_in_stock];
    	if ($isinstock === true) {
    		$i++;
    	}
    }
    if ($i == 0) {
    		echo '<div class="circle">Product is out of stock</div>';
    }
    
}

Getting PHP Notice: Undefined variable: i