1 files changed, 2 insertions, 1 deletions

diff --git a/src/openssl.c b/src/openssl.c
index e146098..a73917a 100644
--- a/src/openssl.c
+++ b/src/openssl.c
@@ -1924,8 +1924,9 @@ static int bn__shr(lua_State *L) {
 
 static int bn__unm(lua_State *L) {
         BIGNUM *a = checksimple(L, 1, BIGNUM_CLASS);
+        BIGNUM *r = bn_dup(L, a);
 
-        BN_set_negative(a, !BN_is_negative(a));
+        BN_set_negative(r, !BN_is_negative(a));
 
         return 1;
 } /* bn__unm() */