diff --git a/bignum.c b/bignum.c
index 3052cbe3..37b2de8e 100644
--- a/bignum.c
+++ b/bignum.c
@@ -1347,7 +1347,7 @@ sexp sexp_sub (sexp ctx, sexp a, sexp b) {
     r = sexp_fx_sub(a, b);      /* VM catches this case */
     break;
   case SEXP_NUM_FIX_FLO:
-    r = sexp_make_flonum(ctx, sexp_fixnum_to_double(a)-sexp_flonum_value(b));
+    r = sexp_make_flonum(ctx, a==SEXP_ZERO ? -sexp_flonum_value(b) : sexp_fixnum_to_double(a)-sexp_flonum_value(b));
     break;
   case SEXP_NUM_FIX_BIG:
     tmp1 = sexp_fixnum_to_bignum(ctx, a);
diff --git a/vm.c b/vm.c
index cdb6ef9f..f97ed0c7 100644
--- a/vm.c
+++ b/vm.c
@@ -1717,9 +1717,9 @@ sexp sexp_apply (sexp ctx, sexp proc, sexp args) {
     else if (sexp_flonump(tmp1) && sexp_flonump(tmp2))
       _ARG1 = sexp_fp_sub(ctx, tmp1, tmp2);
     else if (sexp_flonump(tmp1) && sexp_fixnump(tmp2))
-      _ARG1 = sexp_make_flonum(ctx, sexp_flonum_value(tmp1) - (double)sexp_unbox_fixnum(tmp2));
+      _ARG1 = sexp_make_flonum(ctx, sexp_flonum_value(tmp1) - sexp_fixnum_to_double(tmp2));
     else if (sexp_fixnump(tmp1) && sexp_flonump(tmp2))
-      _ARG1 = sexp_make_flonum(ctx, (double)sexp_unbox_fixnum(tmp1) - sexp_flonum_value(tmp2));
+      _ARG1 = sexp_make_flonum(ctx, tmp1==SEXP_ZERO ? -sexp_flonum_value(tmp2) : sexp_fixnum_to_double(tmp1)-sexp_flonum_value(tmp2));
 #endif
     else sexp_raise("-: not a number", sexp_list2(ctx, tmp1, tmp2));
 #endif