Invalid Email with add to cart button

[Kyouma_okabe]

I am trying to make an 'add to cart' button with a sandbox account.

This is the code I've generated:

<form target="paypal" action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_s-xclick">
<input type="hidden" name="hosted_button_id" value="5RLWTS6RSXEHN">
<input type="image" src="https://www.sandbox.paypal.com/en_US/i/btn/btn_cart_SM.gif" border="0" name="submit" alt="PayPal - The safer, easier way to pay online!">
<img alt="" border="0" src="https://www.sandbox.paypal.com/en_US/i/scr/pixel.gif" width="1" height="1">
</form>

And this is the error I get when testing it out. What have a done wrong?