diff --git a/opt/bignum.c b/opt/bignum.c
index 77619d2e..f68f4821 100644
--- a/opt/bignum.c
+++ b/opt/bignum.c
@@ -554,7 +554,7 @@ sexp sexp_sub (sexp ctx, sexp a, sexp b) {
     r = sexp_bignum_normalize(r);
     break;
   case SEXP_NUM_FLO_FIX:
-    r = sexp_make_flonum(ctx, sexp_fixnum_to_double(b)+sexp_flonum_value(a));
+    r = sexp_make_flonum(ctx, sexp_flonum_value(a) - sexp_fixnum_to_double(b));
     break;
   case SEXP_NUM_FLO_FLO:
     r = sexp_fp_sub(ctx, a, b);
diff --git a/tests/r5rs-tests.scm b/tests/r5rs-tests.scm
index 41919386..048c36d7 100644
--- a/tests/r5rs-tests.scm
+++ b/tests/r5rs-tests.scm
@@ -205,6 +205,8 @@
 
 (test -3 (- 3))
 
+(test -1.0 (- 3.0 4))
+
 (test 7 (abs -7))
 
 (test 1 (modulo 13 4))