diff --git a/system/libraries/Email.php b/system/libraries/Email.php
index 1f2f6a9..011a88c 100644
--- a/system/libraries/Email.php
+++ b/system/libraries/Email.php
@@ -2234,15 +2234,18 @@
 	 */
 	protected function _mime_types($ext = '')
 	{
-		static $mimes;
+		static $_mimes;
 
 		$ext = strtolower($ext);
 
-		if ( ! is_array($mimes))
+		if (empty($_mimes))
 		{
-			$mimes =& get_mimes();
+			// references cannot be directly assigned to static variables, so we use an array
+			$_mimes[0] =& get_mimes();
 		}
 
+		$mimes =& $_mimes[0];
+
 		if (isset($mimes[$ext]))
 		{
 			return is_array($mimes[$ext])