--- osip_message_to_str.c_orig	2024-01-16 01:47:09.570691640 +0100
+++ osip_message_to_str.c	2024-01-16 01:47:24.261373658 +0100
@@ -646,7 +646,7 @@
       if (boundary) {
         /* Needs at most 77 bytes,
            last realloc allocate at least 100 bytes extra */
-        message = osip_str_append(message, boundary);
+        message = osip_str_append(message, boundary+((it.pos)?0:2));
         message = osip_strn_append(message, OSIP_CRLF, 2);
       }