php - Error when passing input variable through to URL string

one text

I feel like I'm missing something extremely simple here...but I'm getting an undefined index

<form action="purchase-checkout.php?MFLHistoryID=<?php echo $_GET["os0"] ?>" method="POST">
        <p>
                <input name="os0" id="os0" maxlength="5" type="text">
        </p>    
        <p>
                <button type="submit" id="checkout-button">Purchase now</button>
        </p>
</form>