diff -r 8a81c5a1b60f src/lib-storage/index/mbox/mbox-sync-rewrite.c
--- a/src/lib-storage/index/mbox/mbox-sync-rewrite.c	Tue Jun 18 18:56:15 2013 +0300
+++ b/src/lib-storage/index/mbox/mbox-sync-rewrite.c	Mon Jun 24 13:12:50 2013 +0300
@@ -19,6 +19,7 @@
 	struct ostream *output;
 	off_t ret;
 
+	i_assert(source > 0 || (dest != 1 && dest != 2));
 	i_assert(size < OFF_T_MAX);
 
 	if (size == 0 || source == dest)