diff --git a/lib/outputlib.php b/lib/outputlib.php
index 53e99076304..416b9445721 100644
--- a/lib/outputlib.php
+++ b/lib/outputlib.php
@@ -823,10 +823,12 @@ class theme_config {
     protected function css_files_get_contents($file, array $keys, css_optimiser $optimiser = null) {
         global $CFG;
         if (is_array($file)) {
+            // We use a separate array to keep everything in the exact same order.
+            $return = array();
             foreach ($file as $key=>$f) {
-                $file[$key] = $this->css_files_get_contents($f, array_merge($keys, array($key)), $optimiser);
+                $return[clean_param($key, PARAM_SAFEDIR)] = $this->css_files_get_contents($f, array_merge($keys, array($key)), $optimiser);
             }
-            return $file;
+            return $return;
         } else {
             $contents = file_get_contents($file);
             $contents = $this->post_process($contents);
@@ -922,7 +924,6 @@ class theme_config {
                 }
             }
         }
-
         return $js;
     }