--- a/master/mailbox.c	Mon Sep 29 10:17:33 2008 +0000
+++ b/master/mailbox.c	Tue Sep 30 07:34:55 2008 +0000
@@ -100,6 +100,7 @@
     if (ec_datagram_fprd(datagram, slave->station_address, 0x808, 8))
         return -1;
 
+    ec_datagram_zero(datagram);
     return 0;
 }
 
@@ -128,7 +129,10 @@
 {
     if (ec_datagram_fprd(datagram, slave->station_address,
                          slave->sii.tx_mailbox_offset,
-                         slave->sii.tx_mailbox_size)) return -1;
+                         slave->sii.tx_mailbox_size))
+        return -1;
+
+    ec_datagram_zero(datagram);
     return 0;
 }