diff --git a/src/lisp.h b/src/lisp.h
index 4487948b007..bf928f51b17 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -4301,7 +4301,8 @@ extern void mark_fns (void);
 
 /* Defined in sort.c  */
 extern void tim_sort (Lisp_Object, Lisp_Object, Lisp_Object *, const ptrdiff_t,
-		      bool);
+		      bool)
+  ARG_NONNULL ((3));
 
 /* Defined in floatfns.c.  */
 verify (FLT_RADIX == 2 || FLT_RADIX == 16);
diff --git a/src/sort.c b/src/sort.c
index 808cd187dcf..02dad327cd4 100644
--- a/src/sort.c
+++ b/src/sort.c
@@ -38,8 +38,6 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 static void
 reverse_slice(Lisp_Object *lo, Lisp_Object *hi)
 {
-    eassert (lo && hi);
-
     --hi;
     while (lo < hi) {
         Lisp_Object t = *lo;
@@ -1095,7 +1093,7 @@ tim_sort (Lisp_Object predicate, Lisp_Object keyfunc,
   Lisp_Object *allocated_keys = NULL;
   merge_state ms;
 
-  if (reverse)
+  if (reverse && 0 < length)
     reverse_slice (seq, seq + length);    /* preserve stability */
 
   if (NILP (keyfunc))