diff --git a/program/include/rcube_imap_cache.php b/program/include/rcube_imap_cache.php
index 6db295ef3..1a442e06f 100644
--- a/program/include/rcube_imap_cache.php
+++ b/program/include/rcube_imap_cache.php
@@ -1146,3 +1146,6 @@ class rcube_imap_cache
         return $index;
     }
 }
+
+// for backward compat.
+class rcube_mail_header extends rcube_message_header { }
diff --git a/program/include/rcube_imap_generic.php b/program/include/rcube_imap_generic.php
index 4a71cd9bb..959dd9fd0 100644
--- a/program/include/rcube_imap_generic.php
+++ b/program/include/rcube_imap_generic.php
@@ -26,9 +26,6 @@
  +-----------------------------------------------------------------------+
 */
 
-// for backward compat.
-class rcube_mail_header extends rcube_message_header { }
-
 
 /**
  * PHP based wrapper class to connect to an IMAP server