diff --git a/master/fsm_slave_scan.c b/master/fsm_slave_scan.c
index 66a7795d..643419c6 100644
--- a/master/fsm_slave_scan.c
+++ b/master/fsm_slave_scan.c
@@ -776,6 +776,23 @@ void ec_fsm_slave_scan_state_sii_data(ec_fsm_slave_scan_t *fsm
                 " Disabling mailbox communication.");
     }
 
+    // clear mailbox settings if invalid values due to invalid sii file
+    if ((slave->sii.boot_rx_mailbox_offset == 0xFFFF) ||
+        (slave->sii.boot_tx_mailbox_offset == 0xFFFF) ||
+        (slave->sii.std_rx_mailbox_offset  == 0xFFFF) ||
+        (slave->sii.std_tx_mailbox_offset  == 0xFFFF)) {
+        slave->sii.boot_rx_mailbox_offset = 0;
+        slave->sii.boot_tx_mailbox_offset = 0;
+        slave->sii.boot_rx_mailbox_size = 0;
+        slave->sii.boot_tx_mailbox_size = 0;
+        slave->sii.std_rx_mailbox_offset = 0;
+        slave->sii.std_tx_mailbox_offset = 0;
+        slave->sii.std_rx_mailbox_size = 0;
+        slave->sii.std_tx_mailbox_size = 0;
+        slave->sii.mailbox_protocols = 0;
+        EC_SLAVE_ERR(slave, "Unexpected mailbox offset in SII data.\n");
+    }
+
     if (slave->sii_nwords == EC_FIRST_SII_CATEGORY_OFFSET) {
         // sii does not contain category data
         fsm->state = ec_fsm_slave_scan_state_end;