diff --git a/core/ProxyHttp.php b/core/ProxyHttp.php
index cb1fa654d37030e8f3219da21b5e4cc89dcdb989..9cd32ea35d073ff27a7da9c031325d0c60c491a2 100644
--- a/core/ProxyHttp.php
+++ b/core/ProxyHttp.php
@@ -239,7 +239,7 @@ class ProxyHttp
      * Returns a formatted Expires HTTP header for a certain number of days in the future. The result
      * can be used in a call to `header()`.
      */
-    private function getExpiresHeaderForFutureDay($expireFarFutureDays)
+    private static function getExpiresHeaderForFutureDay($expireFarFutureDays)
     {
         return "Expires: " . gmdate('D, d M Y H:i:s', time() + 86400 * (int)$expireFarFutureDays) . ' GMT';
     }
@@ -284,4 +284,4 @@ class ProxyHttp
 
         file_put_contents($compressedFilePath, $data);
     }
-}
\ No newline at end of file
+}