diff --git a/include/extern.h b/include/extern.h
index d4a4b432d..0a2fa788d 100644
--- a/include/extern.h
+++ b/include/extern.h
@@ -1,4 +1,4 @@
-/* NetHack 3.6	extern.h	$NHDT-Date: 1573869062 2019/11/16 01:51:02 $  $NHDT-Branch: NetHack-3.6 $:$NHDT-Revision: 1.740 $ */
+/* NetHack 3.6	extern.h	$NHDT-Date: 1573940539 2019/11/16 21:42:19 $  $NHDT-Branch: NetHack-3.6 $:$NHDT-Revision: 1.741 $ */
 /* Copyright (c) Steve Creps, 1988.				  */
 /* NetHack may be freely redistributed.  See license for details. */
 
@@ -450,6 +450,7 @@ E char *FDECL(Amonnam, (struct monst *));
 E char *FDECL(a_monnam, (struct monst *));
 E char *FDECL(distant_monnam, (struct monst *, int, char *));
 E char *FDECL(mon_nam_too, (struct monst *, struct monst *));
+E char *FDECL(minimal_monnam, (struct monst *, BOOLEAN_P));
 E char *FDECL(rndmonnam, (char *));
 E const char *FDECL(hcolor, (const char *));
 E const char *NDECL(rndcolor);
diff --git a/src/do_name.c b/src/do_name.c
index 999fe2055..c8965a706 100644
--- a/src/do_name.c
+++ b/src/do_name.c
@@ -1,4 +1,4 @@
-/* NetHack 3.6	do_name.c	$NHDT-Date: 1560611967 2019/06/15 15:19:27 $  $NHDT-Branch: NetHack-3.6 $:$NHDT-Revision: 1.149 $ */
+/* NetHack 3.6	do_name.c	$NHDT-Date: 1573940540 2019/11/16 21:42:20 $  $NHDT-Branch: NetHack-3.6 $:$NHDT-Revision: 1.150 $ */
 /* Copyright (c) Stichting Mathematisch Centrum, Amsterdam, 1985. */
 /*-Copyright (c) Pasi Kallinen, 2018. */
 /* NetHack may be freely redistributed.  See license for details. */
@@ -1978,6 +1978,42 @@ struct monst *mon, *other_mon;
     return outbuf;
 }
 
+/* for debugging messages, where data might be suspect and we aren't
+   taking what the hero does or doesn't know into consideration */
+char *
+minimal_monnam(mon, ckloc)
+struct monst *mon;
+boolean ckloc;
+{
+    struct permonst *ptr;
+    char *outbuf = nextmbuf();
+
+    if (!mon) {
+        Strcpy(outbuf, "[Null monster]");
+    } else if ((ptr = mon->data) == 0) {
+        Strcpy(outbuf, "[Null mon->data]");
+    } else if (ptr < &mons[0]) {
+        Sprintf(outbuf, "[Invalid mon->data %s < %s]",
+                fmt_ptr((genericptr_t) mon->data),
+                fmt_ptr((genericptr_t) &mons[0]));
+    } else if (ptr >= &mons[NUMMONS]) {
+        Sprintf(outbuf, "[Invalid mon->data %s >= %s]",
+                fmt_ptr((genericptr_t) mon->data),
+                fmt_ptr((genericptr_t) &mons[NUMMONS]));
+    } else if (ckloc && ptr == &mons[PM_LONG_WORM]
+               && level.monsters[mon->mx][mon->my] != mon) {
+        Sprintf(outbuf, "%s <%d,%d>",
+                mons[PM_LONG_WORM_TAIL].mname, mon->mx, mon->my);
+    } else {
+        Sprintf(outbuf, "%s%s <%d,%d>",
+                mon->mtame ? "tame " : mon->mpeaceful ? "peaceful " : "",
+                mon->data->mname, mon->mx, mon->my);
+        if (mon->cham != NON_PM)
+            Sprintf(eos(outbuf), "{%s}", mons[mon->cham].mname);
+    }
+    return outbuf;
+}
+
 /* fake monsters used to be in a hard-coded array, now in a data file */
 STATIC_OVL char *
 bogusmon(buf, code)
diff --git a/src/steed.c b/src/steed.c
index c7540a28a..2f09e5896 100644
--- a/src/steed.c
+++ b/src/steed.c
@@ -1,4 +1,4 @@
-/* NetHack 3.6	steed.c	$NHDT-Date: 1559670610 2019/06/04 17:50:10 $  $NHDT-Branch: NetHack-3.6 $:$NHDT-Revision: 1.66 $ */
+/* NetHack 3.6	steed.c	$NHDT-Date: 1573940541 2019/11/16 21:42:21 $  $NHDT-Branch: NetHack-3.6 $:$NHDT-Revision: 1.67 $ */
 /* Copyright (c) Kevin Hugo, 1998-1999. */
 /* NetHack may be freely redistributed.  See license for details. */
 
@@ -742,6 +742,8 @@ place_monster(mon, x, y)
 struct monst *mon;
 int x, y;
 {
+    struct monst *othermon;
+    const char *monnm, *othnm;
     char buf[QBUFSZ];
 
     buf[0] = '\0';
@@ -749,8 +751,8 @@ int x, y;
        vault guards (either living or dead) are parked at <0,0> */
     if (!isok(x, y) && (x != 0 || y != 0 || !mon->isgd)) {
         describe_level(buf);
-        impossible("trying to place monster at <%d,%d> mstate:%lx on %s",
-                    x, y, mon->mstate, buf);
+        impossible("trying to place %s at <%d,%d> mstate:%lx on %s",
+                   minimal_monnam(mon, TRUE), x, y, mon->mstate, buf);
         x = y = 0;
     }
     if (mon == u.usteed
@@ -762,10 +764,12 @@ int x, y;
                    mon->mstate, buf);
         return;
     }
-    if (level.monsters[x][y]) {
+    if ((othermon = level.monsters[x][y]) != 0) {
         describe_level(buf);
-        impossible("placing monster over another at <%d,%d>, mstates:%lx %lx on %s?",
-                    x, y, level.monsters[x][y]->mstate, mon->mstate, buf);
+        monnm = minimal_monnam(mon, FALSE);
+        othnm = (mon != othermon) ? minimal_monnam(othermon, TRUE) : "itself";
+        impossible("placing %s over %s at <%d,%d>, mstates:%lx %lx on %s?",
+                   monnm, othnm, x, y, othermon->mstate, mon->mstate, buf);
     }
     mon->mx = x, mon->my = y;
     level.monsters[x][y] = mon;