diff --git a/src/Asset/CssOptimizer.php b/src/Asset/CssOptimizer.php
index e942011..40c6b5f 100644
--- a/src/Asset/CssOptimizer.php
+++ b/src/Asset/CssOptimizer.php
@@ -111,11 +111,7 @@ class CssOptimizer extends AssetOptimizer {
    */
   protected function updateUrls($contents, $path) {
     // Determine the file's directory.
-    $directory = '/' . dirname($path);
-
-    // If the file is in the current directory, make sure '.' doesn't appear in
-    // the url() path.
-    $directory = $directory == '.' ? '' : $directory . '/';
+    $directory = base_path() . dirname($path);
 
     // Alter all internal url() paths. Leave external paths alone. We don't need
     // to normalize absolute paths here because that will be done later.