редактирование внешнего ключа

Почему я впервые редактирую внешний ключ, он позволяет мне редактировать его значение и то после второй попытки появляется такая ошибка

Cannot add or update a child row: a foreign key constraint fails (`sadsystem/products`, CONSTRAINT `products_ibfk_3` FOREIGN KEY (`size_id`) REFERENCES `product_sizes` (`size_id`) ON DELETE CASCADE ON UPDATE CASCADE)

, вы можете увидеть мой запрос редактирования

if($mode=="editproduct") 
{
$product_id=$_GET["product_id"];
$sql="SELECT * FROM products WHERE product_id='$product_id'";
$result=mysql_query($sql,$connection) or die(mysql_error());
while($row=mysql_fetch_array($result)) {
$product_id=$row['product_id'];
$product_name=$row['product_name'];
$product_color=$row['product_color'];
$size_id=$row['size_id'];
$product_description=$row['product_description'];
$brand_id=$row['brand_id'];
$category_id=$row['category_id'];
$supplier_id=$row['supplier_id'];
$product_standardPrice=$row['product_standardPrice'];
$product_unitPrice=$row['product_unitPrice'];
} ?>
<link href="default.css" rel="stylesheet" type="text/css">
<form method="post" action="forms.php">
<table align="center">
<tr>
<td><strong>Edit Product</strong></td>
<td><input type="hidden" name="product_id" value="<? echo $product_id ;?>" /></td>
</tr>
<tr>
<td>Product Name</td>
<td><input type="text" name="product_name" value="<? echo $product_name ;?>" /></td>
</tr>
<tr>
<td>Color</td>
<td><input type="text" name="product_color" value="<? echo $product_color ;?>" /></td>
</tr>
<tr>
<td>Size</td>
<td>
<?
    $query="SELECT * FROM product_sizes ORDER BY size_id ASC";
    $result = mysql_query ($query);
    echo "<select name=size_id>";
    while($nt=mysql_fetch_array($result))
    {
    echo "<option value=$nt[$size_id]>$nt[size_name]</option>";
    }
    echo "</select>";
?>
</td>
</tr>
<tr>
<td>Description</td>
<td><input type="text" name="product_description" value="<? echo $product_description ;?>" /></td>
</tr>
<tr>
<td>Brand</td>
<td>
<?
    $query="SELECT * FROM brands ORDER BY brand_name ASC";
    $result = mysql_query ($query);
    echo "<select name=brand_id>";
    while($nt=mysql_fetch_array($result))
    {
    echo "<option value=$nt[brand_id]>$nt[brand_name]</option>";
    }
    echo "</select>";
?>
</td>
</tr>
<tr>
<td>Category</td>
<td>
<?
    $query="SELECT * FROM categories ORDER BY category_name ASC";
    $result = mysql_query ($query);
    echo "<select name=category_id>";
    while($nt=mysql_fetch_array($result))
    {
    echo "<option value=$nt[category_id]>$nt[category_name]</option>";
    }
    echo "</select>";
?>
</td>
</tr>
<tr>
<td>Supplier</td>
<td>
<?
    $query="SELECT * FROM suppliers ORDER BY supplier_name ASC";
    $result = mysql_query ($query);
    echo "<select name=supplier_id>";
    while($nt=mysql_fetch_array($result))
    {
    echo "<option value=$nt[supplier_id]>$nt[supplier_name]</option>";
    }
    echo "</select>";
?>
</td>
</tr>
<tr>
<td>Standard Price</td>
<td><input type="text" name="product_standardPrice" value="<? echo $product_standardPrice ;?>"/></td>
</tr>
<tr>
<td>Unit Price</td>
<td><input type="text" name="product_unitPrice" value="<? echo $product_unitPrice ;?>" /></td>
</tr>
<tr>
<td><input type="submit" name="editproduct" value="Save" /></td>
</tr>
</table>
</form>
<? }