diff --git a/H316/h316_mt.c b/H316/h316_mt.c
index 442cc37a..98d028b4 100644
--- a/H316/h316_mt.c
+++ b/H316/h316_mt.c
@@ -551,6 +551,9 @@ switch (st) {
     case MTSE_WRP:                                      /* write protect */
         mt_err = 1;                                     /* error */
         return STOP_MTWRP;
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }
 
 return SCPE_OK;
diff --git a/I1401/i1401_mt.c b/I1401/i1401_mt.c
index 0b1f55db..925da9cc 100644
--- a/I1401/i1401_mt.c
+++ b/I1401/i1401_mt.c
@@ -443,6 +443,9 @@ switch (st) {
 
     case MTSE_WRP:                                      /* write protect */
         return STOP_MTL;
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }
 
 return SCPE_OK;
diff --git a/I7094/i7094_mt.c b/I7094/i7094_mt.c
index 924a485d..cbd4b2af 100644
--- a/I7094/i7094_mt.c
+++ b/I7094/i7094_mt.c
@@ -797,6 +797,9 @@ switch (st) {
 
     case MTSE_OK:                                       /* no error */
         break;
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }
 
 return SCPE_OK;
diff --git a/Interdata/id_mt.c b/Interdata/id_mt.c
index 0d320145..8421221a 100644
--- a/Interdata/id_mt.c
+++ b/Interdata/id_mt.c
@@ -454,6 +454,9 @@ switch (st) {
     case MTSE_BOT:                                      /* reverse into BOT */
         uptr->UST = uptr->UST | STA_EOT;                /* set err */
         break;
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }                                               /* end switch */
 
 return SCPE_OK;
diff --git a/NOVA/nova_mta.c b/NOVA/nova_mta.c
index 15822877..71439d8e 100644
--- a/NOVA/nova_mta.c
+++ b/NOVA/nova_mta.c
@@ -552,6 +552,9 @@ switch (st) {
         mta_upddsta (uptr, uptr->USTAT | STA_WLK | STA_RDY);
         mta_sta = mta_sta | STA_ILL;                    /* illegal operation */
         break;
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }
 
 return SCPE_OK;
diff --git a/PDP11/pdp11_tm.c b/PDP11/pdp11_tm.c
index af532c14..789086c8 100644
--- a/PDP11/pdp11_tm.c
+++ b/PDP11/pdp11_tm.c
@@ -572,6 +572,9 @@ switch (st) {
     case MTSE_WRP:                                      /* write protect */
         tm_sta = tm_sta | STA_ILL;                      /* illegal operation */
         break;
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }
 
 return SCPE_OK;
diff --git a/PDP11/pdp11_tq.c b/PDP11/pdp11_tq.c
index 01460dcb..c0f007b5 100644
--- a/PDP11/pdp11_tq.c
+++ b/PDP11/pdp11_tq.c
@@ -1585,6 +1585,9 @@ switch (st) {
 
     case MTSE_LEOT:
         return ST_LED;
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }
 
 return ST_SUC;
diff --git a/PDP11/pdp11_ts.c b/PDP11/pdp11_ts.c
index 710e2453..e60433cf 100644
--- a/PDP11/pdp11_ts.c
+++ b/PDP11/pdp11_ts.c
@@ -500,6 +500,9 @@ switch (st) {
     case MTSE_WRP:                                      /* write protect */
         msgxs0 = msgxs0 | XS0_WLE | XS0_NEF;            /* can't execute */
         return (XTC (XS0_WLE | XS0_NEF, TC3));
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }
 
 return 0;
diff --git a/PDP18B/pdp18b_mt.c b/PDP18B/pdp18b_mt.c
index 7656e61e..b320d606 100644
--- a/PDP18B/pdp18b_mt.c
+++ b/PDP18B/pdp18b_mt.c
@@ -472,6 +472,9 @@ switch (st) {
     case MTSE_WRP:                                      /* write protect */
         mt_sta = mt_sta | STA_ILL | STA_ERR;            /* illegal operation */
         break;
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }
 
 return SCPE_OK;
diff --git a/PDP8/pdp8_mt.c b/PDP8/pdp8_mt.c
index 9d65e988..d1fec899 100644
--- a/PDP8/pdp8_mt.c
+++ b/PDP8/pdp8_mt.c
@@ -590,6 +590,9 @@ switch (st) {
     case MTSE_WRP:                                      /* write protect */
         mt_sta = mt_sta | STA_ILL | STA_ERR;            /* illegal operation */
         break;
+
+    default:                                            /* shouldn't happen */
+        return SCPE_IERR;
         }
 
 return SCPE_OK;