diff --git a/PDP1/pdp1_dt.c b/PDP1/pdp1_dt.c
index f9bd69a5..fd8a20ae 100644
--- a/PDP1/pdp1_dt.c
+++ b/PDP1/pdp1_dt.c
@@ -1,6 +1,6 @@
 /* pdp1_dt.c: 18b DECtape simulator
 
-   Copyright (c) 1993-2015, Robert M Supnik
+   Copyright (c) 1993-2017, Robert M Supnik
 
    Permission is hereby granted, free of charge, to any person obtaining a
    copy of this software and associated documentation files (the "Software"),
@@ -25,6 +25,7 @@
 
    dt           Type 550/555 DECtape
 
+   09-Mar-17    RMS     Fixed dt_seterr to handle nx unit select (COVERITY)
    28-Mar-15    RMS     Revised to use sim_printf
    21-Dec-06    RMS     Added 16-channel SBS support
    23-Jun-06    RMS     Fixed conflict in ATTACH switches
@@ -856,16 +857,17 @@ return SCPE_OK;
 
 void dt_seterr (UNIT *uptr, int32 e)
 {
-int32 mot = DTS_GETMOT (uptr->STATE);
-
 dtsa = dtsa & ~DTA_STSTP;                               /* clear go */
 dtsb = dtsb | DTB_ERF | e;                              /* set error flag */
-if (mot >= DTS_ACCF) {                                  /* ~stopped or stopping? */
-    sim_cancel (uptr);                                  /* cancel activity */
-    if (dt_setpos (uptr))                               /* update position */
-        return;
-    sim_activate (uptr, dt_dctime);                     /* sched decel */
-    DTS_SETSTA (DTS_DECF | (mot & DTS_DIR), 0);         /* state = decel */
+if (uptr != NULL) {                                     /* valid unit? */
+    int32 mot = DTS_GETMOT (uptr->STATE);               /* get motion */
+    if (mot >= DTS_ACCF) {                              /* ~stopped or stopping? */
+        sim_cancel (uptr);                              /* cancel activity */
+        if (dt_setpos (uptr))                           /* update position */
+            return;
+        sim_activate (uptr, dt_dctime);                 /* sched decel */
+        DTS_SETSTA (DTS_DECF | (mot & DTS_DIR), 0);     /* state = decel */
+        }
     }
 DT_UPDINT;
 return;
diff --git a/PDP18B/pdp18b_dt.c b/PDP18B/pdp18b_dt.c
index 0666e3f6..76bae988 100644
--- a/PDP18B/pdp18b_dt.c
+++ b/PDP18B/pdp18b_dt.c
@@ -1,6 +1,6 @@
 /* pdp18b_dt.c: 18b DECtape simulator
 
-   Copyright (c) 1993-2016, Robert M Supnik
+   Copyright (c) 1993-2017, Robert M Supnik
 
    Permission is hereby granted, free of charge, to any person obtaining a
    copy of this software and associated documentation files (the "Software"),
@@ -27,6 +27,7 @@
                 (PDP-9) TC02/TU55 DECtape
                 (PDP-15) TC15/TU56 DECtape
 
+   09-Mar-17    RMS     Fixed dt_seterr to handle nx unit select (COVERITY)
    10-Mar-16    RMS     Added 3-cycle databreak set/show entries
    07-Mar-16    RMS     Revised for dynamically allocated memory
    13-Mar-15    RMS     Added APIVEC register
@@ -1269,16 +1270,17 @@ return SCPE_OK;
 
 void dt_seterr (UNIT *uptr, int32 e)
 {
-int32 mot = DTS_GETMOT (uptr->STATE);
-
 dtsa = dtsa & ~DTA_STSTP;                               /* clear go */
 dtsb = dtsb | DTB_ERF | e;                              /* set error flag */
-if (mot >= DTS_ACCF) {                                  /* ~stopped or stopping? */
-    sim_cancel (uptr);                                  /* cancel activity */
-    if (dt_setpos (uptr))                               /* update position */
-        return;
-    sim_activate (uptr, dt_dctime);                     /* sched decel */
-    DTS_SETSTA (DTS_DECF | (mot & DTS_DIR), 0);         /* state = decel */
+if (uptr != NULL) {                                     /* valid select? */
+    int32 mot = DTS_GETMOT (uptr->STATE);               /* get motion */
+    if (mot >= DTS_ACCF) {                              /* ~stopped or stopping? */
+        sim_cancel (uptr);                              /* cancel activity */
+        if (dt_setpos (uptr))                           /* update position */
+            return;
+        sim_activate (uptr, dt_dctime);                 /* sched decel */
+        DTS_SETSTA (DTS_DECF | (mot & DTS_DIR), 0);     /* state = decel */
+        }
     }
 DT_UPDINT;
 return;