diff -Naur mpfr-2.1.0-p12/sub1.c mpfr-2.1.0-p13/sub1.c
--- mpfr-2.1.0-p12/sub1.c	2005-01-23 23:18:41.000000000 +0000
+++ mpfr-2.1.0-p13/sub1.c	2005-02-02 14:13:48.000000000 +0000
@@ -100,7 +100,7 @@
 	     So if we round to Zero, we have to remove one ulp.
 	     Otherwise the result is correctly rounded. */
 	  if (MPFR_IS_LIKE_RNDZ (rnd_mode, MPFR_IS_NEG (a))) {
-	    mpfr_nexttoward (a, c);
+	    (MPFR_IS_POS (a) ? mpfr_nextbelow : mpfr_nextabove) (a);
 	    return -MPFR_INT_SIGN (a);
 	  }
 	  return MPFR_INT_SIGN (a);
diff -Naur mpfr-2.1.0-p12/tests/tui_sub.c mpfr-2.1.0-p13/tests/tui_sub.c
--- mpfr-2.1.0-p12/tests/tui_sub.c	2004-04-05 08:53:04.000000000 +0000
+++ mpfr-2.1.0-p13/tests/tui_sub.c	2005-02-02 14:19:05.000000000 +0000
@@ -1,6 +1,6 @@
 /* Test file for mpfr_ui_sub.
 
-Copyright 2000, 2001, 2002, 2003, 2004 Free Software Foundation.
+Copyright 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation.
 
 This file is part of the MPFR Library.
 
@@ -129,6 +129,12 @@
   else
     MPFR_ASSERTN(mpfr_cmpabs (x, y) == 0 && mpfr_sgn (x) != mpfr_sgn (y));
 
+  mpfr_set_prec (x, 73);
+  mpfr_set_str_binary (x, "0.1101111010101011011011100011010000000101110001011111001011011000101111101E-99");
+  mpfr_ui_sub (x, 1, x, GMP_RNDZ);
+  mpfr_nextabove (x);
+  MPFR_ASSERTN(mpfr_cmp_ui (x, 1) == 0);
+
   mpfr_clear (x);
   mpfr_clear (y);
   mpfr_clear (res);