diff --git a/doc/fixes3-7-0.txt b/doc/fixes3-7-0.txt
index 6b1696bc8..f0805d252 100644
--- a/doc/fixes3-7-0.txt
+++ b/doc/fixes3-7-0.txt
@@ -937,6 +937,10 @@ the u.ustuck hierarchy is:  swallowed by ustuck, hero poly'd into sticky form
 when a vampire gains levels and grows into a vampire lord, change its cham
 	field (shapechanger base type) from vampire to vampire lord so that if
 	it revives in base form it won't revert to plain vampire
+a migrating long worm that couldn't arrive could be placed at <0,0> while
+	setting up another migration attempt to the level, triggering
+        impossible "trying to place long worm tail at <0,0> mstate:8 on level"
+	(message is confused; it should say "long worm" without tail)
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository
diff --git a/src/do_name.c b/src/do_name.c
index 8febc88fd..7dc882553 100644
--- a/src/do_name.c
+++ b/src/do_name.c
@@ -2186,7 +2186,7 @@ minimal_monnam(struct monst *mon, boolean ckloc)
         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]
+    } else if (ckloc && ptr == &mons[PM_LONG_WORM] && mon->mx
                && g.level.monsters[mon->mx][mon->my] != mon) {
         Sprintf(outbuf, "%s <%d,%d>",
                 pmname(&mons[PM_LONG_WORM_TAIL], Mgender(mon)),
diff --git a/src/dog.c b/src/dog.c
index ee8e635cc..d4be1a07d 100644
--- a/src/dog.c
+++ b/src/dog.c
@@ -609,8 +609,10 @@ mon_leave(struct monst *mtmp)
            more segments than can fit in that field gets truncated */
         num_segs = min(cnt, MAX_NUM_WORMS - 1);
         wormgone(mtmp);
-        /* put the head back */
-        place_monster(mtmp, mx, my);
+        /* put the head back; note: mtmp might not be on the map if this
+           is happening during a failed attempt to migrate to this level */
+        if (mx)
+            place_monster(mtmp, mx, my);
     }
 
     return num_segs;
diff --git a/src/mon.c b/src/mon.c
index cfae1214e..6d781bad9 100644
--- a/src/mon.c
+++ b/src/mon.c
@@ -3293,6 +3293,12 @@ m_into_limbo(struct monst *mtmp)
 static void
 migrate_mon(struct monst *mtmp, xchar target_lev, xchar xyloc)
 {
+    if (!mtmp->mx) {
+        /* this was a failed arrival attempt from a prior migration;
+           force mtmp to temporarily have a valid location when starting
+           its new migration */
+        mtmp->mx = u.ux, mtmp->my = u.uy;
+    }
     unstuck(mtmp);
     mdrop_special_objs(mtmp);
     migrate_to_level(mtmp, target_lev, xyloc, (coord *) 0);