diff --git a/sexp2bignum.c b/sexp2bignum.c
index 69757cee98ffa7b09e377368f6b1faabcb55c1e1..f7ef68843f9feb6003c403d01776234503442578 100644
--- a/sexp2bignum.c
+++ b/sexp2bignum.c
@@ -38,10 +38,12 @@ nettle_mpz_set_sexp(mpz_t x, unsigned limit, const struct sexp_iterator *i)
       && i->atom_length
       && !i->display)
     {
-      if (limit && (8 * i->atom_length > limit))
+      /* Allow some extra here, for leading sign octets. */
+      if (limit && (8 * i->atom_length > (16 + limit)))
 	return 0;
       
       nettle_mpz_set_str_256_s(x, i->atom_length, i->atom);
+      /* FIXME: How to interpret a limit for negative numbers? */
       return !limit || mpz_sizeinbase(x, 2) <= limit;
     }
   else