PHP Fatal error: Call to undefined function download_url()

First, Here is the full error:

PHP Fatal error:  Call to undefined function download_url() in /path/to/wordpress/wp-admin/includes/media.php on line 562

I have also posted the related functions at the bottom of my question.

I am modifying a script for my company’s website which allows us to automate the retrieval and uploading of content from our content provider. Due to the way they organize their XML, I need to upload the images related to the articles separately from the main body of the articles. I tried using media_sideload_image() to do this, but I recieved the error:

The function (media_sideload_image()) which calls download_url and results in the error:

 PHP Fatal error:  Call to undefined function download_url() in /path/to/wordpress/wp-admin/includes/media.php on line 562

As can be seen, I successfully included media.php – and the rest of my script has already been implemented on to the site, and I have not run into any other issues with accessing WordPress files. The error would appear to be with media.php itself – which I find unusual.

Any ideas on how I can resolve this problem? Alternatively, if you know of another function I can use to do this, that would also be appreciated.

By all means, if you require any more details on the issue just ask.

/**
 * Download an image from the specified URL and attach it to a post.
 *
 * @since 2.6.0
 *
 * @param string $file The URL of the image to download
 * @param int $post_id The post ID the media is to be associated with
 * @param string $desc Optional. Description of the image
 * @return string|WP_Error Populated HTML img tag on success
 */
function media_sideload_image($file, $post_id, $desc = null) {
    if ( ! empty($file) ) {
        // Download file to temp location
        $tmp = download_url( $file );

        // Set variables for storage
        // fix file filename for query strings
        preg_match('/[^?]+.(jpg|JPG|jpe|JPE|jpeg|JPEG|gif|GIF|png|PNG)/', $file, $matches);
        $file_array['name'] = basename($matches[0]);
        $file_array['tmp_name'] = $tmp;

        // If error storing temporarily, unlink
        if ( is_wp_error( $tmp ) ) {
            @unlink($file_array['tmp_name']);
            $file_array['tmp_name'] = '';
        }

        // do the validation and storage stuff
        $id = media_handle_sideload( $file_array, $post_id, $desc );
        // If error storing permanently, unlink
        if ( is_wp_error($id) ) {
            @unlink($file_array['tmp_name']);
            return $id;
        }

        $src = wp_get_attachment_url( $id );
    }

    // Finally check to make sure the file has been saved, then return the html
    if ( ! empty($src) ) {
        $alt = isset($desc) ? esc_attr($desc) : '';
        $html = "<img src='$src' alt='$alt' />";
        return $html;
    }
}

The function where I call media_sideload_image() in my code:

//Upload the image if it exists, and return the post_id
function upload_image($post_id, $image_url) {
    require_once('wp-admin/includes/media.php');
    $image_url = 'http://admin.gkbusiness.com/gkbusiness/files/2011/04/LOGOGKMBUS1.jpg';
    media_sideload_image($image_url, $post_id);
    return $post_id;
}