diff --git a/sys/kern/subr_rman.c b/sys/kern/subr_rman.c
--- a/sys/kern/subr_rman.c
+++ b/sys/kern/subr_rman.c
@@ -445,6 +445,8 @@
 	       "length %#jx, flags %x, device %s\n", rm->rm_descr, start, end,
 	       count, flags,
 	       dev == NULL ? "<null>" : device_get_nameunit(dev)));
+	KASSERT(count != 0, ("%s: attempted to allocate an empty range",
+	    __func__));
 	KASSERT((flags & RF_FIRSTSHARE) == 0,
 	    ("invalid flags %#x", flags));
 	new_rflags = (flags & ~RF_FIRSTSHARE) | RF_ALLOCATED;
@@ -520,7 +522,7 @@
 		DPRINTF(("truncated region: [%#jx, %#jx]; size %#jx (requested %#jx)\n",
 		       rstart, rend, (rend - rstart + 1), count));
 
-		if ((rend - rstart + 1) >= count) {
+		if ((rend - rstart) >= (count - 1)) {
 			DPRINTF(("candidate region: [%#jx, %#jx], size %#jx\n",
 			       rstart, rend, (rend - rstart + 1)));
 			if ((s->r_end - s->r_start + 1) == count) {