diff --git a/opt/bignum.c b/opt/bignum.c
index 8cbd012a..9af36997 100644
--- a/opt/bignum.c
+++ b/opt/bignum.c
@@ -143,7 +143,7 @@ double sexp_bignum_to_double (sexp a) {
   sexp_uint_t *data=sexp_bignum_data(a);
   for (i=sexp_bignum_hi(a)-1; i>=0; i--)
     res = res * ((double)SEXP_UINT_T_MAX+1) + data[i];
-  return res;
+  return res * sexp_bignum_sign(a);
 }
 
 sexp sexp_bignum_fxadd (sexp ctx, sexp a, sexp_uint_t b) {