Akismet fatal error

I’ve been using Akismet for many years and never had a problem. But now all of a sudden comments on my site are broken. Any time someone tries to post one, we get the message:

Fatal error: Call to undefined function set_url_scheme() in /home/covermes/public_html/wp-content/plugins/akismet/class.akismet.php on line 840

I know a lot of people use Akismet, has anyone encountered this? I tried reinstalling the plugin, which didn’t work. I’ve pasted the file below, with line 840 in bold:

// Check if SSL requests were disabled fewer than X hours ago.
$ssl_disabled = get_option( ‘akismet_ssl_disabled’ );

if ( $ssl_disabled && $ssl_disabled < ( time() – 60 * 60 * 24 ) ) { // 24 hours
$ssl_disabled = false;
delete_option( ‘akismet_ssl_disabled’ );
}
else if ( $ssl_disabled ) {
do_action( ‘akismet_ssl_disabled’ );
}

if ( ! $ssl_disabled && function_exists( ‘wp_http_supports’) && ( $ssl = wp_http_supports( array( ‘ssl’ ) ) ) ) {
$akismet_url = set_url_scheme( $akismet_url, ‘https’ );

do_action( ‘akismet_https_request_pre’ );
}

$response = wp_remote_post( $akismet_url, $http_args );

Akismet::log( compact( ‘akismet_url’, ‘http_args’, ‘response’ ) );