Checkout / Unexpected token

The problem arises when the customer tries to proceed from step 4 (Shipping) to Step 5 (payment method) does anyone know the solution to the problem?

SyntaxError: Unexpected token <
OK
<b>Notice</b>: Undefined index: comment in <b>/home/******/public_html/********.com/catalog/controller/checkout/shipping_method.php</b> on line <b>131</b>[]

Thanks!