diff --git a/opt/bignum.c b/opt/bignum.c
index 7e48466c..835b354f 100644
--- a/opt/bignum.c
+++ b/opt/bignum.c
@@ -564,7 +564,7 @@ sexp sexp_sub (sexp ctx, sexp a, sexp b) {
     r = sexp_bignum_normalize(sexp_bignum_sub(ctx, NULL, a, tmp));
     break;
   case SEXP_NUM_BIG_FLO:
-    r = sexp_make_flonum(ctx, sexp_flonum_value(b) - sexp_bignum_to_double(a));
+    r = sexp_make_flonum(ctx, sexp_bignum_to_double(a) - sexp_flonum_value(b));
     break;
   case SEXP_NUM_BIG_BIG:
     r = sexp_bignum_normalize(sexp_bignum_sub(ctx, NULL, a, b));