Fatal error: Call to undefined function wp_unslash() …..

This error appears when I try to upgrade to 3.6.

Here is the area of code referenced:

<?php
foreach ( (array) $extra_fields as $field ) {
if ( isset( $_POST[ $field ] ) )
echo ‘<input type=”hidden” name=”‘ . esc_attr( $field ) . ‘” value=”‘ . esc_attr( wp_unslash( $_POST[ $field ] ) ) . ‘” />’;
}
submit_button( __( ‘Proceed’ ), ‘button’, ‘upgrade’ );
?>

I’m not a php expert so could use some help, please!

What’s the problem and what’s the fix?

Site is http://savings4.us but it is functioning OK.