diff --git a/cdn.module b/cdn.module
index 4e19013..80437f5 100644
--- a/cdn.module
+++ b/cdn.module
@@ -67,6 +67,14 @@ function cdn_file_url_alter(&$path) {
   global $user;
 
   if ($status == CDN_ENABLED || ($status == CDN_TESTING && user_access(CDN_PERM_ACCESS_TESTING))) {
+
+    // Convert a public streamwrapper path into its external path.
+    $scheme = file_uri_scheme($path);
+    if ($scheme == 'public') {
+      $wrapper = file_stream_wrapper_get_instance_by_scheme($scheme);
+      $path = $wrapper->getDirectoryPath() . '/' . drupal_encode_path(str_replace('\\', '/', (file_uri_target($path))));
+    }
+
     // If the current path is an absolute path, return immediately.
     $fragments = parse_url($path);
     if (isset($fragments['host'])) {