diff --git a/VAX/is1000_sysdev.c b/VAX/is1000_sysdev.c
index 69635516..543d341e 100644
--- a/VAX/is1000_sysdev.c
+++ b/VAX/is1000_sysdev.c
@@ -898,7 +898,7 @@ char gbuf[CBUFSIZE];
 
 get_glyph (ptr, gbuf, 0);                           /* get glyph */
 if (gbuf[0] && strcmp (gbuf, "CPU"))
-    return SCPE_ARG;                                /* Only can specify CPU device */
+    return sim_messagef (SCPE_ARG, "Invalid boot device: %s, must specify BOOT CPU or simply BOOT\n", gbuf);
 return run_cmd (flag, "CPU");
 }
 
diff --git a/VAX/vax410_sysdev.c b/VAX/vax410_sysdev.c
index 575f242d..eb08882d 100644
--- a/VAX/vax410_sysdev.c
+++ b/VAX/vax410_sysdev.c
@@ -836,7 +836,7 @@ char gbuf[CBUFSIZE];
 
 get_glyph (ptr, gbuf, 0);                           /* get glyph */
 if (gbuf[0] && strcmp (gbuf, "CPU"))
-    return SCPE_ARG;                                /* Only can specify CPU device */
+    return sim_messagef (SCPE_ARG, "Invalid boot device: %s, must specify BOOT CPU or simply BOOT\n", gbuf);
 return run_cmd (flag, "CPU");
 }
 
diff --git a/VAX/vax43_sysdev.c b/VAX/vax43_sysdev.c
index d4746ca0..84b67653 100644
--- a/VAX/vax43_sysdev.c
+++ b/VAX/vax43_sysdev.c
@@ -957,7 +957,7 @@ char gbuf[CBUFSIZE];
 
 get_glyph (ptr, gbuf, 0);                           /* get glyph */
 if (gbuf[0] && strcmp (gbuf, "CPU"))
-    return SCPE_ARG;                                /* Only can specify CPU device */
+    return sim_messagef (SCPE_ARG, "Invalid boot device: %s, must specify BOOT CPU or simply BOOT\n", gbuf);
 return run_cmd (flag, "CPU");
 }
 
diff --git a/VAX/vax440_sysdev.c b/VAX/vax440_sysdev.c
index d1f9dba2..c7fd84f7 100644
--- a/VAX/vax440_sysdev.c
+++ b/VAX/vax440_sysdev.c
@@ -954,7 +954,7 @@ char gbuf[CBUFSIZE];
 
 get_glyph (ptr, gbuf, 0);                           /* get glyph */
 if (gbuf[0] && strcmp (gbuf, "CPU"))
-    return SCPE_ARG;                                /* Only can specify CPU device */
+    return sim_messagef (SCPE_ARG, "Invalid boot device: %s, must specify BOOT CPU or simply BOOT\n", gbuf);
 return run_cmd (flag, "CPU");
 }
 
diff --git a/VAX/vax630_sysdev.c b/VAX/vax630_sysdev.c
index 14dc4785..c73e9316 100644
--- a/VAX/vax630_sysdev.c
+++ b/VAX/vax630_sysdev.c
@@ -903,7 +903,7 @@ if ((ptr = get_sim_sw (ptr)) == NULL)               /* get switches */
     return SCPE_INVSW;
 get_glyph (ptr, gbuf, 0);                           /* get glyph */
 if (gbuf[0] && strcmp (gbuf, "CPU"))
-    return SCPE_ARG;                                /* Only can specify CPU device */
+    return sim_messagef (SCPE_ARG, "Invalid boot device: %s, must specify BOOT CPU or simply BOOT\n", gbuf);
 return run_cmd (flag, "CPU");
 }
 
diff --git a/VAX/vax_sysdev.c b/VAX/vax_sysdev.c
index 2f46e572..efb328f2 100644
--- a/VAX/vax_sysdev.c
+++ b/VAX/vax_sysdev.c
@@ -1700,7 +1700,7 @@ if ((ptr = get_sim_sw (ptr)) == NULL)               /* get switches */
     return SCPE_INVSW;
 get_glyph (ptr, gbuf, 0);                           /* get glyph */
 if (gbuf[0] && strcmp (gbuf, "CPU"))
-    return SCPE_ARG;                                /* Only can specify CPU device */
+    return sim_messagef (SCPE_ARG, "Invalid boot device: %s, must specify BOOT CPU or simply BOOT\n", gbuf);
 return run_cmd (flag, "CPU");
 }