diff --git a/lib/hcrypto/bn.h b/lib/hcrypto/bn.h
index 43692ec97..924ccf9ce 100644
--- a/lib/hcrypto/bn.h
+++ b/lib/hcrypto/bn.h
@@ -97,7 +97,7 @@ int	BN_num_bytes(const BIGNUM *);
 int	BN_cmp(const BIGNUM *, const BIGNUM *);
 
 void	BN_set_negative(BIGNUM *, int);
-int	BN_is_negative(BIGNUM *);
+int	BN_is_negative(const BIGNUM *);
 
 int	BN_is_bit_set(const BIGNUM *, int);
 int	BN_set_bit(BIGNUM *, int);